big_ben said:No one is paying attention to me saying this but I'll say it again. The best mod to do if you don't want to void the warranty is to get a 3" exhaust made from the turbo back. This will let it spool faster, and it will also slightly raise the boost without touching the turbo or IC pipes. I will almost guarantee you will gain more than 10hp at the wheels if you get a 3" exhaust.
*COUGH* I said it (werd)
Also based on the current setup and possible drive-train loss, I am thinking a new downpipe/exhaust with a high-flow cat should get 15-20, maybe more depending on the built in restrictions in the stock setup and the ability for the computer to relearn the new setup.