with a lighter pully you are lightening the rotainal mass. there is a formula for the exzact % ratio`s. but it is something like every 1 lb of rotainal mass that is lost, is = to almost 4-5 whp...

you could do a replacement flywheel that is very close to the stock weight, just a little bit lighter. not 50% lighter. and since it is closer to the stock weight you would be able to have a beefier flywheel. but personally i have nerver had one. but i have seen a good improvemt with the perrin pully. but it is something that you might have to tweek a little something here and there with your tune.