I think the Stage I Rousch is rated at 315hp. Someone else here posted that even though it has an aftermarket exhaust there isn't any change in hp.
A GTO will crush a Stage I or II Rousch Mustang. Stage III can present a challenge to a GTO. They can have a supercharger on them, but not always. With the supercharger they are rated at something just north of 400. I saw a road test and it was in the low 13's. I raced a guy with a Stage III supercharged at a local strip. I beat him by .2 and 1mph even though he had a slightly better 60'. I was playing catch-up till the 1000' mark. It was a hot summer hight and he was running 13.6-13.8 versus my 13.3-13.6 runs.