The following link has some really good information about the effects of lightening a flywheel.

Light-weight flywheel information

Enjoy

 

Ohh yeah rotational mass is everything compared to throwing weight off your car. I plan on buying a lightweight flywheel and better clutch. Its just like throwing 8 lbs off of each wheel. It makes a huge difference. Thats why alot of people are going with 16" wheels instead of 18's. Good link though and that should help alot of people out.