Something that in all these months had oddly escaped my notice:
The text of the Spell Improvisation Virtue says that the virtue does not stack with other Casting Total bonuses. I utterly fail to understand the sense of this ? What does it mean, you cannot apply a bonus from knowing a similar spell if you apply a CT bonus from the other gazillion possible sources (vis, focus, potent, diedne, chtonic, theurgy, talisman, astrology...), or the other way around, you cannot apply these bonuses if have this one. But it seems idiotic, either way !! Either the virtue is almost useless, or it is really a Flaw !
Hm, I don't actually remember why that's there. I suspect I was thinking that you can't artificially increase the level of the spell to get a bigger bonus, though I don't know how you could do that exactly. Adaptive Casting, maybe? I seem to remember that in an earlier draft Faerie Raised Magic did the same thing without including the same Virtue, so I was probably trying to prevent magi with both from double dipping.
I don't think it should be read as meaning you can't use Spell Improvisation with a magical focus, vis, or Diedne Magic, though. Those bonuses make sense to me. I'd say you just shouldn't be able to stack it with anything that basically gives a bonus for knowing a similar spell.
Ahh, I see. Just another case of rushed last-minute changes and conflicting mix of evolving playtest versions casually getting stuck in a rather confusing way in final print, like an insect in amber. OK. Thanks for the clarification, now the virtue makes sense again.
I think this one should be earmarked for the HoH:MC errata.