Well I am having a Performance shop install a SLP Cat Back on my 07 Grand Prix GT Friday. I purchased it from SLP and they drop shipped it to the shop. The guy at the shop said it would open up my exhaust and i would pick up about 10 HP or so. He even mentioned it was possible to gain a few MPG. Now one thing I've been told the OEM mufflers aren't that restricting but the pipe is. Factory pipes have some minor crimps in them and they are not stainless. Most of us here use mods like CAI, FWI, Pulleys, Headers and Cat Backs, and of course PCM tuning. All these mods add some Horse power to the wheels, crank whatever. I've been hearing some horrors stories from mechanics about PCM tuning though. The PCM can support some changes from mods like CAI, Exhaust changes, T-stat, colder plugs and even the SC pulley as long as you don't try to run one the size of a quarter. I like having a good running car and some of these mods are nice and they do add some power. But the only way you truly make a car fast is to go into the engine and transmission not the PCM or the exhaust or change T-stat and spark plugs. Those are minimal changes that your PCM can deal with. Ya want 500 HP at the wheels do all those simple mods if ya want then find a real sharp cat that can get into your engine and tranny. I've heard some guys say on here they've blown engines because of so and so's PCM tuning. I would honestly say that the popular online PCM guys and I don't have to mention who they are do I. Probably spend 15-30 min with your PCM and then ship it back to ya and the ones off the shelf, don't expect much out of those either. Find someone local who has the equipment and knows your vehicle and let him spend a few hours refining your PCM's parameters. I think all mods have some value in terms of adding power to a car but be cautious how far you go. I don't race my car, I'm willing to bet most of you don't either ya just want a street beast, nothing wrong with that really but make it a stable one. I can go on and on with this but I think ya all get it.
