as stated above, the stock cat back flows good until 350 WHP, so there will be no performance improvements for that mod. Forget the catbacks unless you are doing it for sound. If you want it just to sound good, save some money and just order some borla mufflers to put on it. That will improve sound and save you money. Personally, I would go with the 3" DP and that will give you a little bit of a deeper tone as well, along with performance gains....