A lighter rotator may accelerate a little quicker, but won't make much (if any) more hp. Gains from "adding lightness" in and of itself are small. However, lighter components in the rotating assy. may allow more rpm and thus more power.
there are guys in super stock who switched to the honda journal stuff with a new crank and all,,, on a small chevy it ended up being 10lbs lighter from crank, and rods,, that equated to about .03 in a 10.0 sec car
Bought one of the Superlight cranks from Scat back in 2006, 3.75 stroke with the pendulum undercut counterweights. Sure looked purdy, but other than taking maybe 5 to 10 pounds off the front wheels, and making it easy to lift on and off the engine balancer (had to use a slug of mallory per end) all it really did was lighten my wallet.