My point is that if you step back from the mechanics of the game, then you realize they are both about casting faster.
I could say that Fast Casting is not just about defense, but that would be tweaking you. 
But combining them with something else? Silent multicasting? Silent fast-casting? (I know this last is not allowed as per ArM5 p.83)
Umm, we are discussing mechanics. So...?
Deja vu, all over again. Sure, fast cast that offensive spell, get it off twice in a round. I don't object to that. I think we know where I have an objection. 
Now. If that person can't combine Quick casting and Fast casting...
Sure, why not? Marcus wants to silently and still cast 6 Balls of Abysmal Flame... There is a combat advantage to that...
And I'd argue with the silent fast casting a bit. Does someone with Quiet and Still casting need to make gestures for fast casting when they don't have to otherwise?
My point exactly, combining them easily quickly becomes overly powerful. If Synthesis is the ability to combine different masteries, I think it should get harder to combine them the more you want to do with each of them.
Let's take Marcus (to take the name of a character that is not from this saga) and say he has Mutiple Casting 6, Fast Casting, Quick Casting 6, and Synthesis 4.
As you present it, he can multi-cast 6 Balls of Abysmal Flame as a fast-cast action, with a +6 to his fast-cast speed.
What I'm proposing is that he has to choose, if he wants to combine his masteries. He could use each mastery individually, for example multi-cast 6 BoAF, or cast a single one at +6 Initiative. Or he could multi-cast 2 of them, as a fast-cast action, with a +2 to his fast-cast speed (so 5 points of mastery used).
He still has a lot of tactical flexibility because of his multiple masteries, but combining them is a bit harder. I think that's more balanced. Also more colorful and dramatic, as you have to make a choice each time, instead of falling back to "throw everything at once".
Well, 7 BoAF.
Discounting Flawless Magic, that's...
265 experience points: Multicasting 105 + Quick Casting 105 + Fast Casting 5 + Synthesis 50.
If you've spent that much XP in something, why wouldn't it be easy to combine that? I mean, that's a lifetime of effort there.
And if I were to speculate as to how it would apply to Flawless, which I have thought about, it sticks with the individual ability, meaning you can't spend across abilities within the same spell. It would be 134: Multicasting 53 + Quick Casting 53 + Fast Casting 3 + Synthesis 25, which is still a mess of XPs...
Under RAW, you get synthesis automatically, under my proposal, I'm just asking for it to be paid for (to still be possible).
I know you're saying you still pay for it, you just get less bang for the buck. Maybe Synthesis instead allows up to X number of additional mastery levels to be chosen beyond the first?
So Marcus could cast 7 Ballsy of Abysmal flame with a fast cast action and a +3 to his fast cast speed. Remember, Synthesis should always be N+1, because by default you can already do one. If you have Synthesized Casting 1, under your example, you don't get any benefit...
Actually, you do, because I did use Synthesis Score +1. So with a score of 1, you can fast-cast 2 BoAF.
It's all a matter of taste. I dislike the thought of 7 fast-cast BoAF with +6 to casting speed and +6 to effective Penetration Score, no matter how much xp it takes to get it.
Along those lines, it might be better to simply keep RAW mastery, except that Multiple Casting must be bought once for each additional copy you want to be able to cast. But I'm sure that will not be to your tastes. 
Sorry, missed that one earlier. Quick Casting and Fast Casting are both about casting fast, simply put. So why did they go with separate mechanics? It simply doesn't make sense, and I think that was a mistake. Merging them would allow a single ability about speed, that can be increased, instead of two different abilities, one of which provides a big benefit at the first level but with no further use as an ability.
So I'd rather go with: Provides the ability to fast-cast, as well as a fast-casting speed bonus or initiative bonus equal to the (score - 1).
In the same way, I'd merge Rebuttal and Unravelling, which are basically the same thing for different Techniques + Vim.
Would you object to Praxiteles being able to get a +6 to Finesse, use Quiet Casting x 2 for a Synthesis of 1?
Is Multiple casting the issue (I think it is, it's probably way too powerful, regardless). Maybe it needs to be ability score/2 to determine the number of extra copies?
The stuff about Quick and Fastcasting being merged is fine by me...
Well, any high score with a single mastery being easy to combine with something else makes me edgy. So yes, I'd object. Not a showstopper objection, but this simply makes be feel "blah" when I look at the combo.
The more I think about it, the more I like RAW with simply a modification to multiple casting (one additional copy per time you select it)... and perhaps those masteries that use the whole score instead of +X per time taken (Apotropaic, Imperturbable, Penetration, Rebuttal and Unraveling).
BTW, another way to fix the power of multiple casting would be to say that you must divide your casting bonus between the number of copies cast. You still roll for each seperately. The magus only has so much magical energy to work with, after all.
My goal, for Praxiteles is to get to a very decent score to Precise casting, so that he can boost finesse for the spells that allow sculpting, primarily. Ideally, he'd like to do quiet casting x2. Under Raw, that's a total ability score of 8, which is...expensive, but I could probably live with it.
Why would he need Quiet Casting for those spells, really? He could just take the -10 to his casting total for casting them silently. Most of the spells won't have that high a level, unless I'm mistaken, so with reasonable scores (10-15) in Rego and Terram it won't be too much of an issue. Consider the fact his mastery score of 6 would also add to his casting bonus for the spell.
Well, it's his living in the heart of the city, so to speak. Speaking Ancient Greek...might seem odd. I dunno. Thinking out loud, doesn't really matter one way or another, in a saga like this.
So, after all this discussion, are we making any change to mastery? I'd like to hear what the others think as well, so let's give them a chance to pipe in on this.
I'd be on board with making everything...equal.
Either Precise or Quick casting are based on the score, or not. If they aren't based on the score, then multiple casting and all other abilities that are based on the score need to be bought x number of times.
It just needs to be one way or the other...
On a related note, I have added a page on the wiki to summarize the House Rules (referenced in the main page). I have reworded some of them for better readability, so give them a look at let me know if I made any mistake.
The rules about mastery are still in draft, with a note at the beginning to clearly state this.
Regarding Mercurian Magic and the automatic learning of Wizard's Communion.
Considering that we will use the variant Ritual Communion for rituals, I am wondering if a Mercurian may also learn Wizard's Communion as well, at the same level, whenever he learns a second ritual spell of the same level. After all, the virtue does state that mercurians are especially skilled in "magic used in conjunction with others", not just ritual spells.
I would infer that, when a Mercurian learns a first level 20 ritual, he automatically learns Ritual Communion at level 20. Upon learning a second level 20 ritual, he would automatically learn Wizard's Communion at level 20. (We could also have him learn both when he learns the initial ritual, but that seems abusive to me.)
Would that make sense?
Note that since Petronius knows 2 level 20 rituals at saga start, this means he would know both Communion spells at level 20 as well.
Well, I think the Ritual Communion is enough. He can invent a Wizard's Communion from first principles easily enough (I'd say Ritual Communion qualifies for the similar spell bonus, at full magnitude), and the magic of the Cult of Mercury seemed largely ritualistic in nature...
Well, that is why I quoted from the virtue's description:
Mercurian are described as not just skilled at ritual magic, but also skilled at working with others.
Considering how weak the virtue is for a Major one, I don't think it would be unreasonable. IMO, Mercurians should not have to learn a Wizard's Communion spell by itself, ever. That's one of the (few) benefits from Mercurian Magic.
And I'd argue that the magic of the Cult of Mercury wasn't so much ritualistic in the Hermetic understanding of the term (requiring vis), as ceremonial and collaborative (takes a lot of time and many participants).
I do recall suggesting that we adopt the House Rule in Bibracte...
Mercurians can cast spontaneous spells normally, but get a bonus if they go ceremonial. Ritual spells are cast if the caster has stalwart mastery.
My above proposal addresses the ceremonial components, at least.
The collaborative components have always been problematic to me, because often times Mercurians of a young age can often cast ritual spells in their specialty well in excess of the Ritual Communion/Wizard Communion that they know and can cast. The spell is actually the least of my concerns, especially when Petronius learns Hermes Portal, boom. He knows a 75th level Ritual Communion. Does he honestly have a chance of casting such a beast? 
No offense meant, but I've answered that proposal by saying that it didn't feel appropriate to me. I'd rather go as close to RAW as possible, to explore all the possibilities of the virtue as written. Stalwart Mastery is not useful to all rituals. And I don't believe it makes sense for a Mercurian to be better at spontaneous magic (even if it is by using ceremonial casting) than a regular Hermetic magus. I find it completely appropriate that the take a lot of time casting spontaneous spells.
A level 75 Wizard's Communion? No chance at all, ever. Even his current level 20 version of the spell is a struggle at the moment.
Which is a bit what my proposal is about. Basically, I think that, every time a Mercurian learns a new ritual, he should be able to learn a new version of Wizard's Communion (or Ritual Communion), at whatever level he wants. Or perhaps gain xp toward mastery of Wizard's Communion? Mastery of WC actually provides a benefit to a Mercurian, as the mastery score adds to the effective level of the Communion. And these make sense under RAW, as Mercurians are supposed to be skilled with communion.
In short, whenever a new ritual is learned, he gets the choice to do one of the following:
Just thinking out loud here...
Absolutely none taken. I just find the idea that he can keep adding Wizard's Communion spells at will whenever he invents a new ritual spell. It is one of those things that breaks my suspension of disbelief.
I'd be more on board with this than getting a new spell for the first two rituals created at a higher level. And it solves the problem of the 75th level Wizard/Ritual Communion spell that will never be cast, let alone doesn't explain how it's learned. Although the idea that 10 wizards combine to work together to cast each individual's Ritual communion does have some appeal to it...
Works for me