Could you mount the header with a 2.5" carsound cat to a stock GSR catback/exhaust?? will i get good gains or should i just buy an aftermarket exhaust system.

 

since the jdm 4-2-1 header will be longer, it will not fit with a usdm spec cat. I believe smsp sells something so you can use your usdm exhaust with a jdm header and it will bolt up fine. He makes the cat shorter i think.