2.5" exhaust pip or not

Just stuck on APN "header" and doin' a flowmaster 40 muffler. Stock pipes still.
Will it make that much of a dif if I put a 2.5" tailpipe on the muffler? I mean power sound and mileage differences as opposed to leavin' on the stock pipe out of the muffler. I only have time to do it today so any thought greatly appreciated. hanks lol:wink:

I've got a 92 4.0 and i've got it straight piped coming out of the converter with stock manifold on and exiting in front of the rear passenger tire. Once I pulled off the pipes and muffler and straight piped it of course it made it alot louder but it felt like it gave it more power due to less exhaust restriction. Now straight piping prolly won't be emissions legal where you live but I'd say best to go to the 2.5 inch pipe as it will increase the tone depth some and might help a little with power.