The current rule is both fairly clear and fairly restrictive. You must use firm voice and bold gestures, and anything that is optional cannot be used. If that is more restrictive than desired, then the wording really should be changed. In any change I would be careful about allowing things that clearly should take too much time. And I wouldn't drop the current rule entirely, as I don't think we're supposed to be able to combine fast casting and multiple casting, use vis, use flourishes, nor use forceless casting. Even if the virtues Quiet Magic, Subtle Magic, and Deft Form are allowed, I'm not sure that should mean Quiet Casting and Still Casting should necessarily be allowed. I would tend to agree with what lvgreen said:
I think it's also worth noting that Slow Caster is a Minor Flaw that is already pretty hefty without stopping fast casting. But it explicitly does prevent true fast casting (as those take a full round). If this Minor Flaw prevents fast casting this way, is it really too much to penalize some other Flaws, especially Major Flaws, similarly?
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Yes, the whole end of the round thing and what timing you have to beat has always been questionable. Happily, we can also set a reasonable value easily enough.
It is a little vague, but I really don't think just piling on Pilum of Fire fits the description:
A maga may choose to cast a Spontaneous spell extremely quickly, as a response to an attack or other surprising event.
I'm pretty sure "or other surprising event" is supposed to be like roof collapsing on you or similar non-attack. Also, if this is allowed for attacks, it breaks several rules in the game. One case is immediate: if I want as fast-cast defense against your fast-cast Pilum on a non-initiative, what initiative do I roll against? Another is how dimicatio works. There may be more. Throwing in several offensive spells like this is what Multiple Casting is for.