What about multicasting mastery?
I have my Pilum of Fire mastered for multicasting (and resistance), and cast that spell 3 times as a single action.
I define my MuVi spell as having T: Group (See Group Wizard's Boost, MoH, p. 113), completely changing... what? Presumably all 3 pila?
And by then, I have certainly claimed the mastery ability from my PoF, no?
Obviously.
I'm not really seeing the need to Spontaneously cast the MuVi spell to make it more powerful in practice.
If you have a high level MuVi and the ability to cast it without difficulty, you may be limited to the effect of the MuVi (less versatility i agree) but you can have various copies of the spell with different effects (and in lab you gain the similar spell bonuses etc etc).
for the group MuVi, i'd say it could work, but how can you multi cast 3 pila and the MuVi in one round ? (to my interpretation that's not really possible)
Spell Mastery: multiple casting. Multiple copies of the spell are made for each casting if you invoke the mastery ability. I do it all the time with my weather maga and lightning bolts. I also have a T: Group version (also mastered with multiple casting), and a T:Group, Size+1 for target rich environments.
Edit: I initially misread this. To clarify, multiple casting is still casting one spell, you've just mastered it to create additional copies at the time of casting. If you believe you can cast a single spell in one round and fast cast a MuVi spell to transform it, it's not any different than a multiple cast spell and T:Group version of the MuVi spell to transform it.
I never saw that spell from MoH before and I can't figure out what rulebook guideline it works from. It looks like someone just made it up on the fly.
The basic Wizard's Boost from the corebook works off the "significantly changes" guideline rather than the "totally changes" guideline. I don't think I'd have a problem with keeping mastery bonuses under this circumstance.