Here's a Dyno which was made on a Scuderia in 2010. I'm pretty much sure that these figures has been improved since Capristo considerably improved his exhaust system and especially the cats.
As Russ mentionned, the best and most usefull improvement is in the midrange where torque values increased to 502Nm available from 4263rpm to 5250rpm while stock it is max 467Nm at only 5335rpm. This is the real power you will feel pushing your car much harder. On my F430, I'm getting now the max torque value of 504Nm available from 3822rpm up to 5300rpm. My car is a different car since and is so much more enjoyable with this extra torque and hp in this midrange.
These are real figures which say everything
Compare this with other exhaust brands dynos and you can
make up your mind what you really going to get for your money
Convert the graph from nm to ft. Lbs. http://www.mr2ownersclub.com/converter.htm
495 nm equals 365.04 ft. lbs. An increase of only 18 over the factory 347. So for 15g's, you get 15-18 hp increase, and 18 ft. lbs. Torque increase?? No way that is worth it. Especially if you can't improve the sound.
My car scares the living shit out of people, I had a line of people waiting for me to fire it up when I left Chateau St. Michelle on Sunday. After my SV is done, I am going to add the Evoms tune, and the Capristo flex pipes. Hopefully I can find a good shop to weld in the stock valves.
Doug, you need to re-think what your goals are. I seriously doubt you will be able to feel a 5% gain.
Fabspeeds setup gains 57 hp at the wheels, and 45 torque with headers, straight pipes, X-pipe exhaust, and Evoms tune. http://www.fabspeed.com/dyno67.html
Vs. Stock: http://www.fabspeed.com/dyno86.html
No wonder Capristo uses nm. Makes it look like a bigger gain than it actually is.
I have talked with the Texas customer with that setup, and his car is tuned for and runs straight race gas. He has had not one issue.