My GTA has the dual cat setup. The I pipe was replaced on it before i bought the car last summer. It is slip fit inside the exit pipe off the dual cats. I measured the outside diameter of the of the I-pipe and it is 2.5" (I've got a section of 2.5" pipe and it is the same size). I thought stock was 2.75"? I can not afford an off the shelf cat back system for the car right now, so I am thinking of picking up a used stock 4th gen cat back system so at least I'll have a mandrel bent 2.75" I-pipe. Any guesses as to how much power the crimped to hell 2.5" pipe is robbing? Think 10-15hp could be had with the stock 4th gen setup?

Thanks