Stock one-piece replacement rotors generally weigh about the same. If they don't, I would suspect they are of inferior quality. To really lose rotor weight, you need to go with a multi-piece rotor that has an aluminum hat. Something like these will save you a pound or two on the fronts and let you keep the stock brake setup.
http://frozenrotors.com/product/fron...pt+sti+models/
These may not be the correct rotors, but call them and they will be able to set you up.
http://www.racingbrake.com/Two-piece...exc-p/2042.htm
Generally the rears are tougher to loose weight on because they usually are all steel or iron because of the need to accommodate the drum shoes for the parking brake.