exhaust pipe diameter for a mildly modified 340

lotta people trying to pick pepper out of fly **** here.

600hp motor loses a few hp/tq, @VOETOM shows 1/10th & 1mph

if you're going from a single or a real crappy dual set up, then sure stepping it up to 2~2.5 with some good mufflers is going to be a marked improvement. but the difference between a 2" system and 2.5" system in a sub 400hp street car? what's the ROI on that? a nickel?