with that set-up, not much. i suggest u get the extractors/headers port, flowed and polished first before you touch the exhaust. bang for your buck and u WILL get gains from the smooth gas flow and some gains in low and mid range then i'd be looking at changing the muffler to a 2.5 inch outlet with the 2.5 inch straight pipe. or if you don't have the money change the muffler first then the cat back. u will loose abit of torque changing the pipe to a 2.5 inch but when hitting lift, its a whole new world good luck edit: cat-back is high flow anyways from memory its 4 inch inlet and 3 inch outlet (correct if wrong)