I agree with WiCKeD. I've put about 1,500 into my exhaust, intake, and PCM on an 04 GT and I've increased my HP by about 50. And I don't even have headers. Make sure with your exhaust you get rid of the factory downpipe and put on an after market one with a high flow cat. I think 3" is the way to go, that's what I have on mine.