I dont recall seeing any dyno results from just an exhaust swap. Currently that is my only performance mod, a full 05 GT catback. I think there is a 10-20 hp gain. Maybe not peak hp, but at some rpm ranges the car pulls much harder. Im not basing this on noise because the car really isnt any louder than when I had just the one GT takeoff muff on. I also have 17in GT rims, tires any smaller would be giving me traction problems when launching from a standstill.
The car sticks to the road really well, I was going into some of those curves faster than I wanted to, and the car was still holding the line I picked with no problems. I will have to practice some before I can really figure out what she can do.
All in all Im very pleased. Im $1900 into the GT bumpers, GT duals, GT rims, GT sways, tint, and leather seat covers. I paid 20k, so Im into this for $21,900 so far.
Ive got a long way to go before I come anywhere near the 3-5k dealer markup on a 05 GT.