There can be an advantage in going up a 1/4" in pipe size because the cat heats the exhaust up significantly. Can't say how much it will help you. I can tell you that a flowmaster will be pretty loud, imho. You might do better with a magnaflow or similar. I'm running a flo-pro right now, which is similar to a flowmaster with headers and a full 2.5" exhaust and I don't like the tone. Sounds good at idle and at low rpm, but when you stomp on it and it winds up to 3k rpm or so, it gets really raspy. With the smaller stock exhaust size and manifolds, it should be quieter, but you might want to listen to one first, or get them to agree to a swap if you don't like the flowmaster.