The only thing that the Fast-Casting mastery does is (1) allows you to cast the formulaic spell under Fast Casting rules and (2) reduces the number of botch dice. Taking it once drops the number of additional botch dice to one, while taking it twice eliminates the additional botch dice for fast casting entirely. Without the Fast Casting mastery, you can only cast a Spontaneous spell. For example: if you have Repel the Wooden Shafts as a Formulaic spell, but don't have the Fast Casting Mastery, then you can only cast Repel the Wooden Shafts as a Spontaneous spell.
Oh, there's the rules...I kept missing the part about post-gauntlet lab activities (and I couldn't decide how to dock the points for doing so). Okay, now that I've found them and read them (have I mentioned that this is my first time? 
), I'm going to go ahead and say "player's choice" but be sure to specify that you're doing so if you're using the lab activity rules. (p. 32, third paragraph, starting with the numbered block in the middle (which seems like reasonable measures for doing the lab stuff). Lose 10 xp per season for each lab activity, up to 30 per year. So, if you're inventing a spell that would take you five seasons to invent, it might be better to just do the point-expenditure route.