Our troupe has found Mercurian Magic to be a little weak to be a major virtue. To give a little more power to it while maintaining the flavor we have agreed to have Mercurian Magi to automatically earn the Spell Mastery Ceremonial Casting (HoH:TL) option when they learn any non-ritual formulaic spell.
It makes Mercurian Magi feel a lot more "ritualist" (our mercurian magus now always try to ceremonial cast whenever he can) without making them overpower and the virtue becomes a little more major this way.
I like the base idea of "Mercurians can use Ceremonial Casting by default". I might be inclined to tweak it to "you can use the Ceremonial Casting option with any formulaic spell" rather than making it via a mastery for a couple of reasons:
Being able to lose the last botch dice in stressful situations is a big gain, and makes it feel like it's treading on Flawless Magic's toes too much
For spells which you want to master, but are unlikely to use Ceremonial Casting (e.g. combat spells), having one level of a useless mastery is a disadvantage as it raises the price of all the masteries you actually want.
Don't forget you have the option of folding Road Magic from The Mysteries: Revised Edition into Mercurian Magic as well. You could also fold in Mystical Choreography from HoH: Societas, as it makes ceremonial spontaneous magic a lot more palatable.
Thank everyone for the feedback. Definitively just adding "You can use Ceremonial Casting in your non ritual formulaic spells" to the Mercurian Magic Virtue description is more elegant and effective that my automatic spell mastery option. I am glad to see others have come to similar options in their sagas.
I will think about the mastery option for rituals! Like flawless magic for rituals only... Sounds very promising!
It doesn't have to double XP, like Flawless Magic . Just even +5xp for learning a ritual, meaning that unless they have some flaw like Unpredictable Magic, they can cast rituals without worrying about botching in a peaceful casting circumstance.
I have always thought that since rituals last long enough to require Concentration rolls (nobody I have played with has had Concentration 4+), the possibility of a momentary lapse of concentration causing a deviation from the "correct" implemetation of the ritual, makes ritual casting inherently stressful.