Hey Nick, what Grill said isnt off base, these 5.3L like to have a little back pressure. Before I put mine on, I had 90% of my exhaust off, and believe it or not (aside from being loud as heck) off the line suffered a bit, however my top end felt better. So, from a light, when towing, too open or too big of exhaust system could hurt ya a bit. I tow alot, and have dual in, dual out Magnaflow knockoff exhaust, but the pipes are not the largest you can get. I put on some bigger tips (looks tougher) but that doesnt increase my flow. So I really feel the increase of power, however it sorta depends on where you want you backpressure/torque curve to improve.
Chadstr