That's a tough one. I think if you call Rick Deman at Deman Motorsports in Tarrytown, he might have a clue, at least in where to send you. The car is great out of the box, and essentially uses a 911 engine variant, so there should be SOME knowledge out there. But...Porsche guys, well, they like to throw money sometimes, and will just replace the whole motor before they will tweak what they'vegot.

Also, Porsches generally don't have achilles heels..they don't yeild gobs of power with just a muffler change.
I'd hit Pelicanparts.com and go to the BBS, look for info there on the track forum and the Boxster forum, and I'd call respected Porsche shops in the area, such as eman.