A guy on the Audi forum did a really in depth study of this and concluded that for Audi sizes (similar to WRX), that the effect of rotating weight on the car was 165% of that of body weight. So if you reduce rotating weight by 10#, it's like reducing body weight by 16.5#.
I still say that best bang for the buck is 16" RS wheels. Cheap, strong and pretty light.
jack