Go with Pacesetter headers and their Off Road Y-pipe. (It should bolt up to your Loudmouth catback without any problems) You will see a gain with headers; the guy doesn't know what he is talking about. If you are in Madison, I would recommend going to Speed Inc. if you don't feel like tackling the install yourself. It isn't that hard and can be done on a Saturday.
99 Z28 - Bolton LS1 on drag radials
10.63 @ 124.73 (1.39 60ft) +2100 DA - SOLD