RE: 75 grain broadhead
It's not totally about speed, although I like what I can get. I have used 75gr Wasps and now am using a 75gr Innerloc, well, for about 4 years now.
Each setup is going to be different. You always have to have enough FOC to balance things out, but the main advantage to lighter heads is that usually they have a narrower cut so don't wind plane as bad. This also allows for more ease in tuning for good flight.
Penetration is not an issue with them. Penetration is a result of total arrow weight X velocity squared. The point, whether it is 75gr or 125 gr is just part of the mass of the arrow and in and of itself has little bearing on penetration so long as the blades are sharp.