Removing rotating mass in the driveline is a good thing for horsepower. However, after installing my aluminum driveshaft I would estimate maybe a tenth gain in the 1/4 - at most.
I think I would gain much more by getting rid of my stock 'pony' wheels and replacing them with a nice set of super lightweight Bogarts!
It's a good mod, but not one that I would do first thing.
E