For what it's worth, my '01 dyno'd at 415 rwhp / 449 rwtq with Corsa 3" catbacks only, and smooth tubes/K&Ns added 10 rwhp and 10 ft-lbs.
Also, when I dyno'd my '97 before and after the 3.55 gear, on the same dyno, SAE corrected, there was about 10 hp and 10 ft-lbs difference (lower with the 3.55 gear).