To get to 550whp you'll need to do an O1E 6spd manual swap with the JHM rebuild kit. This gives you the improved 1st & 2nd gear collars. You'll also need Steve K's O1E swap kit and a ClutchMasters HD clutch. It's now official that Tial 605's fit as Tim Haugan's RS6 ( former Sportec Project 6spd car ) is up and running w/ a pair .

With above mods you'll also need new injectors, fuel pump, Wagner IC's , etc...and most importantly a tune. Maybe he can chime in . Corn juice is your friend for the HP you're looking for.