I posted this a while back in the C5 section but never here. This is a customers car we just tuned. It's an '04 C5 with an intake and headers. The parts were on the car and he brought it to us for a tune. Peak improvements were 10 hp and 28 ft-lbs with average gains of 20 hp and 15 ft-lbs. Final numbers were 336 rwhp and 360 rwtq, all done on a Mustang Dyno 1100SE.
http://lh6.google.com/KaltechTuning/R6Etoa_HsXI/AAAAAAAAARE/5WIziEj5Xbc/d%20chambers.JPG?imgmax=512