???
You'd consider the spell level to stay the same? I've always assumed it goes up, as with flexible formulaic magic. +5 per magnitude.
I think limited arts can be reached using easier methods.
The 3rd edition was the better in this for now. There were no book quality and xp for arts - less bookkeeping. And the system was quite easy. You compared the level with your present art. If the book level was higher you got 1 level, twice higher - 2 levels, 3 times higher - 3 levels. The same with vis learning but you multiplied the roll by 3. And you added the aura I already forget before or after multiplication. There were no tractati.
And the key of the present system are the tractati. Summae help early improvement but tractati alone are responsible for the easy eligible high level arts. So if you erase this rule beginners advancement will be ok but not the elders.
I like this because I think young magi want and can learn more than aged magi.
Maybe learning from vis should be better like multiplying the roll by two.
Nowhere does it say that the extra effects achieved through vis-boosting increase the level of the spell. Rereading the mutantes rules, vis boosting is the ability that I considered more overpowered. The example spells do not take the full potential out of this ability IMO.
The redcaps are also sensibly vis-rich as a house, so this makes the vis availability to those 12 guiys EXTEREMELY high. I would not be surprised to see them having 50-100 pawns each per year as regular income. That makes vis boosting a no brainer.
Remember that each time you memorize a spell you invent it, so you have invented all the spells in your grimmore regardless of who wrote the first version of the spell.
I have found this not to be as powerful as the first time I read it, but I still think that vis boosting can easily be abused. The auto-cancellation one coupled with circle duration versions of waiting spells can make for quite a few nasty tricks being charged in several items carried by the magus and activated by concentrating (no need for voice or gestures. Arm yourself with 100 or 200 instanjt spells inside circle containers and there you go on an adventure where you do not need to cast a single spell, only supress circle containers.
Xavi
Aha. Now I understand how you worked out the forumla. My skill in Jeek Code is a 1 at most, so I didn't undertand the numenclature of the formula 
The numbers in the table are...ugly, though. I would suggest modifying them by hand to make them into round numbers. Multiples of 50 and 100 for the larger numbers, for example, rounding down. It doesn't matter much and the table looks way more neat doing that.
And level 1 should still be 1 XP 
Appart from that, nice rules. Some V&F would need to be modiofyed to fit in (like skilled parens) to justify the loss of XP you are imposing on them, but appart from that it seems it can work really well.
Cheers,
Xavi
All these points about Mutantes Magic are excellent. I must admit, the part about the level of the spell increasing with the boost never occurred to me. Like The Fixer suggested, I think it would work best when the level of the spell increases with the boost, like Flexible Formulaic Magic.
Yes, the spell level stays the same (unfortunately).
FLEXIBLE FORMULAIC MAGIC
Major, Hermetic
You can vary the effects of formulaic spells to a slight degree, while still getting the benefits of casting known magic. You may raise or lower the casting level of the spell by five to raise or lower one (only) of Range, Duration, and Target by one step, as long as this does not violate any of the normal limits on formulaic magic. Casting success, fatigue loss, and Penetration are all calculated based on the casting level of the spell. You cannot manipulate Ritual magic in this way.
Note, specifically, the casting level of a known formulaic spell can be 'modified' and it is additionally clarified so that it is clear that the actual spell level is changing, ie this is a new spell at a higher or lower level that can be cast 'as though' it was a known formulaic spell.
As opposed to Boosting, where as you cast the original spell, you power in some extra-raw-magical-power (vis) and the spells effects balloon out. Its still the original spell being cast though.
BOOSTED MAGIC
Minor, Hermetic
By spending a pawn of vis when casting formulaic spells you may “boost†the Range, Duration, or Target of the effect by one magnitude. You can do this multiple times for the same spell. For example, boosting Range from Touch to Sight and Target from Individual to Group would cost you four pawns of vis. This has no effect on spontaneous or Ritual spells, though you can still use vis to boost your Penetration as normal. Note that while magi with Mutantum Magic (see below) can invent spells that allow this, they cannot boost spells that were not designed to do so without taking this Virtue.
I utterly agree it would be waaaaay less abusable (assuming vis wealth) if it worked in a similar manner to FFM where it effectively became a new spell at a modified level.
One might say that the Mercere have an element of muscle that is all their own, and damn scary to boot. Don't mess with the redcaps!
It is also true that the more powerful the magus the more useful the harnessing and tethering elements become.
As regarding inventing spells, the actual wording for Boosted Magic is that it cannot be used for spells that are not designed to be boosted. Thus inventing a spell off a lab text from a 'normal' spell would not (IMO) count as a 'designed-for-boosting' spell as you are just re-inventing someone elses design.
My mistake in using the term inventing in place of self-designing, sorry. 
shrug if the character wants to burn vis that way, why not let them? It's not like they've got infinite stores of it, right? And remember that vis boosting was standard hermetic magic in the previous editions - it didn't reduce penetration either, though I'll grant you it was more expensive.
remember also that anyone can boost spellcasting with the help of vis. No virtue required - get +2 for each pawn...
As I already said, a gifted redcap is likely to have a minimum vis assortment of 50-100 pawns per year. That is the base "McDonald's level" salary for them. Then they would get also extra vis for house stuff. The redcaps want muscle backup? They get it, and they pay to get it.
A medium redcap can easily boost his spells of death to AC using vis boost. And he is likely to have it available.
Cheers,
Xavi
What leads you to this conclusion?