If anything you'd be releasing extra horsepower that your restrictive factory exhaust was hindering.  It won't be much without replacing the entire exhaust system, but it shouldn't hurt it at all.  Plus, it should sound pretty mean!

Only gain is sound. Won't hurt anything.

True it wont hurt a thing I have a borla exhaust system and it freed up the breathing on my engine. I gained a little bit only because I have a supercharger but replacing the exhaust is the easiest gain on horsepower.

Don't forget an intake system as well, i'd also recommend aftermarket headers while you're at it. i did a full exhaust and intake change on my Sunfire and gained almost 30 more horsepower

Please don't get flowmasters. I have a 99 m6 trans am and am very familiar with ls based engines. They sound
like crap. Go to ls1.com click on sponsors and check out thunder racing and lmperformance. Magnaflow and SLP
are the two best that i know of. The magnaflow you can adjust flow through an exhaust plate and the SLP is one
of the most free flowing exhausts for the ls base.

some people actually do loose power with the flowmaster's

they aren't a very good choice for modern cars.



go to LS2GTO.com to get more info
or LS1tech.com

hell,
if you want some mufflers,   I've got a pair of Magnaflow's (sound better,  and better gain than flowmasters) in my garage,   they already have the flanges and hangers welded on to fit a 2005-06 gto

I put on headers on my 06 and I'm trying to decide if I want to stay with the stock axle-back or go with the Magnaflows.  I almost prefer headers + stock muffler sound to be honest...haven't looked into performance comparo yet.

Magnaflows seem to be the most popular performance replacement choice for the Holden based GTOs.  Spintech is another option that a few seem to like a lot (and quite a few don't, of course).  Flowmasters seem to be the least favorite choice, in my unscientific, general obersavation based opinion.