Scrappy/elirentz - Ferrarri states the electrical system improves the power/weight ratio on the website linked in the first post. It also states the torque from the electrical motor gave them freedom to optimize the V12 for high RPM power delivery, which probably is the main reason this car is going to sound, drive, and be awesome.
I realize it may be all marketing fluff but it seems reasonable to assume that if Ferrarri could make a faster one without this HY-KERS system, they would. We'll see. If this ends up being the best performing Ferrarri, then score one for the hybrid system.