Phil,

I did a little more research and it appears the offroad pipe is indeed 3.0 in diameter (as stated in your emails with Eric Wong of GPS). This will not bolt on nicely with your stock exhaust system.

If you get a 3.0" diamater-piped exhaust system in the future, then you could use the offroad pipe.

Else, as stated by Eric, you could use the offroad pipe as a TRACK MOD only- meaning attach one end of the offroad pipe (closest to engine) and leave the other end OPEN and hanging. (closest to rear of car)

Vortec also brings up some good points about the offroad pipe vs high flow cat. I dont think the GT wants to lose any more low end power if possible.