Hi,
I think Toa nailed it in the very first response. Flawless magic for a single form is a fine minor virtue. It is not the most powerful minor virtue available, perhaps, but is extremely efficient, since it pays to specialize. If you want the virtue to be a little bit more powerful, then have the minor virtue provide flawless magic for a single art.
A specialist is not necessarily all that fragile. A character who start play concentrating in a single form and a single technique is quite goodgood at nearly 30% of all hermetic magic, and has some devastating capabilities in his niche. A minor focus plus 2 affinities with arts plus 2 puissant arts already extremely effective, and a targeted flawless magic ices the cake, leaving points for good parens, affinity with magic theory, and another major hermetic virtue. Or some other fun combination.
Sometimes a major virtue deserves complexity, but in this case I think the simple version is simply better. it is more flexible and is more apt.
The conversation you have later regarding other major virtues is interesting, but I think it misses the mark.
First, comparing hermetic and non-hermetic virtues, and the way they are constructed, is sometimes useful but not always. They tend to be taken by different kinds of characters. I think it is easier to justify the major version of a regular virtue being more than 3 times as good as the minor version, because grog characters cannot have the major version. The major versions being elite to be justified because only the lead characters get to take them.
Second, your analysis of shape changer versus skin changer is dead on, right until you try to reach your favorite conclusion. Another way to look at this is that just as one shape is a minor virtue and all shapes is a major virtue, so to having a major version of flawless magic cover all hermetic magic is completely consistent with the minor version covering a single form. (By the way, neither skin changer or shape changer is a very effective virtue for magi, who can better achieve this effect either through spells or by being a Bjorn air. also, shape changer costs experience points and only covers a limited number of shapes and requires a skill roll.)
Strong faerie blood is an excellent major virtue when used properly. But it is far from overpowering, and many players want their characters to have a touch of faerie get exactly what they want from faerie blood. Is a strong faerie blood more than 3 times as good as faerie blood? Often it isn't. (If the 15 extra years are used for initial skill points, then maybe, but most players don't use it this way, and get all the bang for the buck they need from having a plus 1 presence, or a crafting bonus, and so on. For wizard characters, a small bonus to aging is often inconsequential.)
Magister in arty bus (Dragon has been doing pretty well so far today) is not more than 3 times better than, say, good parens. It is not more than 3 times better than the baccalaureate virtue from arts and academe.
An assemblage of benefits into a major virtue sometimes seems to be better than the cost, but sometimes is not; each needs to be taken on its own merits. For example, many players believe that mercury in magic is not worth the cost of a major virtue. Diedne magic certainly is not: life linked spontaneous magic usually provides a better benefit, and does not come with a major flaw built in. Mythic blood is also probably not worth its cost; the minor focus is balanced by an intrinsic minor flaw, leaving 30 spell levels that don't gain the benefits of your art scores (good parens 30 real spell levels and then adds 60 experience points) and sometimes saves you fatigue, when you try to cast a formulaic spell and missed by a little bit.
By the way, if you want to go for a unique and simple major virtue that is plenty powerful, how about something like this:
mastered form – hermetic major virtue
You have an ability that starts at 1 that acts as mastery for all of your formulaic and ritual spells of a single form. The mastery benefits you choose for each level apply to all applicable spells if it is otherwise legal to do so.
(Change this to "mastered art" if this isn't powerful enough.)
But I still think Toa 's version is better, because it is closest to existing game mechanics and is appropriately priced.
Anyway,
Ken