If a spell has wizard's boost (or wizard's reach or wizard's expansion etc) applied to it, can it have penetration greater than the penetration of the wizard's boost?
In other words, if A pilum of fire is cast with a penetration of 30 and it has a wizard's reach spell cast upon it to give it range sight with the wizard's reach having a penetration of 10, and the spell strikes a target with a magic resistance of 20 does the spell penetrate?
Muto vim (wizard's boost) is momentary .
Muto vim, which target a spell in creation (and not a spell created) is basically turning a formulaic spell into a spontaneous. Spontaneous spell exists by themselves.
by the errata, I would say that yes both have to penetrate.
However I lack to understand how it can work since the Muto Vim spell resulted in a real consequence and I think this errata clearly create a paradox for muto vim magic.*
The "magical energy" you were working (which is not a spell) turned by your muto vim in a spell which is real; when the spell is cast, the "magical energy" is gone. So there is no more "muto" magic. This is enforced by the fact that you are only able to muto vim spell which are not yet cast. A spell not cast is not a spell, but magical energy in process of becoming a spell. PerdoVim can affect both. Muto is said to be imperfect regarding transforming magic.
So, IMO, it's like using a momentary muto spell to transform you in a geant form, breaking chains which were holding you. When the momentary spell is gone, the chains are still destroyed.
If Wizard's Boost is affecting a spell, and the Boost's penetration is insufficient to bypass the magic resistance of the target of the original spell, the original spell is blocked even if it had, on its own, enough penetration to affect the target. This is analogous to a magical pink dot making a mundane sword blockable by magic resistance. "Staining" an opponent's spell with a 0 penetration Vim effect can be an effective defensive strategy.
I'd be more interested in seeing any argument using quotes from the books or just reason. I don't care nearly as much about your position as much as I care about why you hold it,
Exarkun basically put forward my position (the wizard's boost is over before the spell penetrates the target's magic resistance so it shouldn't matter) but the nonsensical "muto vim needs to last as long as the target spell despite the fact that it's based on the final level of the target spell rather than the base level" issue seems to make it into A pink dot issue.
From a gameplay angle you want to decide whether or not you want stuff like pilum of fires and wieldong the invisible sling to be wizard's boosted by characters who aren't exceptionally strong in muto vim. I think that wizard's boosting is never so awsome a choice for a combat situation that it needs to be restricted for fear of abuse. I'd ignore the penetration of the muto vim effect when deciding whether a boosted spell penetrates. I think it would be more fun.
I agree with Erik and Exar here. MuVi targets the casting of a spell not the target of the targeted casting. If the targeted casting is being done by someone else then the MuVi would need to penetrate their MR. As such a momentary duration is sufficient for normal sponts and formulaics that are cast in less than a round. I can see a case for requiring that ceremonial and ritual castings require an extended duration MuVi that covers the whole casting time e.g. concentration. The counter to that is "what if I want to change a ceremony/ritual that has already started? You know that would be cool!" It would be cool - so make it a possible alternative with its own more expensive guideline or have the standard guidelines for a momentary MuVi that takes effect at the climax of the ritual but if one uses a concentration MuVi to play with the gathering energies throughout the extended casting then making the same change to the final effect is much easier. An intellego requisite so one can see what is doing could make it easier still.
I am now picturing a wizard who cons his way into cults and hijacks their rituals 
I think mine was more of a reasoned, though concise, argument than just a statement of my position. I'll try to be clearer.
Muto needs to be sustained to keep affecting the target. The description of the Muto technique is very clear about this. No changes enforced directly on the target by a Muto spell outlast the Muto spell. Muto Vim is no exception to this, as clarified by the errata (which I find very logical and coherent with the other uses of the technique - I'm not sure why you call it nonsensical). If you use a Wizard Boost with Sun duration to affect a Moon duration spell, the Moon duration spell gets "unboosted" when the Wizard Boost expires, just like a grog turned into a giant by a Muto Corpus spell with Sun duration turns back into a normal human when the Muto Corpus spell expires.
Anything currently subjected to a magical effect is stopped by parma or magic resistance stronger than the penetration of the magical effect. This is a general rule. A mundane sword changed by a Muto Terram spell is stopped by magic resistance stronger than the penetration of the Muto spell, even if the sword by itself could have bypassed the magic resistance. The fist of a grog who's been turned into a giant by a Muto Corpus spell is stopped by magic resistance stronger than the penetration of the Muto spell, even if the fist by itseld could have bypassed the magic resistance. A Creo Ignem spell Boosted by a Muto Vim spell is stopped by magic resistance stronger than the penetration of the Muto spell, even if the Creo Ignem spell by itself could have bypassed the magic resistance.
I fail to see what's unclear about this.
Does "cooperating" from the guidelines mean that you reduce your PoF penetration to match the MuVi spell's?
If yes, then you have your answer.
Lets assume a slow Diameter PoF. When the MuVi Duration is over the PoF loses its extra Range (The Sorcerer's Fork). A PeVi could Target the MuVi and cancel the extra Range before it hits the foe.
If the PoF was fired through an Aegis, both would have to survive the boundary. You cannot drink magical water unless it penetrates your Parma. Your MuVi will fizzle on the Parma. Does the PoF fizzle at the exact same time?
If no, it has time to hit your target.
Parma is some distance from the skin to protect your clothes. Fizzle is "shorter" than the distance to your clothes or they would get damaged by blocked spells.
My RAW opinion is: both spells keep their full Penetration, both fizzle at the same time, no damage.
I would houserule to allow it to hit, though.
Because otherwise it would be Rego instead of Muto.
Probably not. As without the effect of the MuVi spell, it couldnt reach, so if it doesnt penetrate, then the modified spell fails to reach further than the Parma.
You are saying that a Muto Vim affects a spell created. Which is not possible: MutoVim has to be cast on spells before they are created.
And something "before its creation" doesn't exist (you can't target Bismark with perdo corpus). So the target of MuVi is not a spell (it doesn't exist). It's magical energy, which ends when the spell is cast, after the effect of MuVi, and thus, the spell is perfectly natural.
The spell being not the target of the MuVi cast before its existence, but only the result of its application to its target (the magical energy worked in a spell during a process called "casting spell"), the MuVi :
As ezzelino pointed out, that contradicts RAW on how Muto works.
Your model makes in game sense, I didn't mean to imply otherwise (although, looking back at post 4, I certainly did , sorry about that). What I find unreasonable is that the spells as written reflect your model. all of the spells in the core book have a duration of momentary.
The errata says:
This means that, rather as implied by the core book spells, being able to invent a spell that allows my character to boost his mentem spells one magnitude I can invent one that allows him to boost only his Momentary mentem spells one magnitude. If I want to invent one that can handle duration moon then the muto vim spell needs to be three magnitudes higher then the target spell, but the MuVi spell also has to also match the level of the target spell the spell has to add an addition three more magnitudes, spell design shouldn't have to pay for the same thing twice.
Also, although there could possibly be some room for argument here, it seems to me that restricting the power of muto vim spells in this fashion sucks a little bit of fun out of the game and adds a little bit more anal retentive rulesyness to it.
What's the upside to your model?
It's logically consistent and consistent with the other muto spells
What's the downside?
Muto vim is severely weakened in that you'll need substantially more spells to do the same things (argue if you want but this looks less fun to me)
There is no good rationale for the restriction of not allowing a muto vim spell to target a spell that 's already cast
MuVi Spells end up paying for duration twice which doesn't make as believable a model
the core book, before the reader checks the errata gives them the wrong idea
the penetration of muto vim spells is reduced in a way that also looks less fun to me.
I think that a better option is to have the muto vim spells alter the formation of the target spells. It's a bit of extra "lore" for the new players to absorb but it resolves the question of why cant muto vim spells target existing spells and it gives results that fit in the range of power that I consider to be fun (neither too powerful nor too weak).
I guess I should have thought through the issue more before I posted the original question and reread the errata. The rules are sufficiently clear to unambiguously answer the question, I didn't see it in part because I didn't want to but also because I didn't double check before I posted.
This is one of those issues that could do with some more clarification in RAW.
The Muto Vim guidlines start out by saying.
Now nominal often means something along the lines "not actual" or "in name only'. So don't you think this sentence could be read as.
The duration of most Muto Vim spells is momentary in name only, and they affect the spell they have altered for it's entire duration.
Of course the errata says what it says so that overrules that interpretation. Or does it? If the duration of most Muto Vim spells is not actually momentary then it must be some special duration. So this special duration is called Momentary, but it lasts as long as the target spell. A stretch yes but any time two rules seem to contradict you either have to toss out one or the other or stretch both till they fit. Some spells like The Sorcerers Fork would still use the standard duration rules for conceptual or Rule One reasons. Also some spells would only need the standard momentary duration because the spell really only effects things important at the moment of casting like Wizards Communion.
This bit of house ruling gives you a Muto Vim thats consistent with other Muto magic, doesn't have to pay for duration twice, explains the need to be cast with the other spell and finally isn't substantially weakened.
As far as penetration goes since the spell isn't actually expired you would have to go with the lower of the two penetrations. However it should be pointed out that you can't pink dot a spell with Muto Vim. If you try to target a hostile spell the rules say you have to beat it's penetration total with your own to even effect it.
And it's a really common way to interpretate legal texts IRL... so valid point.