I would say it SHOULDN'T, but they can get nasty if your car ever has a problem, and try to blame it on the different backpressure. You COULD say you HAD to change it due to failed components in the exhaust, but they'll wonder why you didn't claim it in the first place.
Dunno if your warranty is GM or not, but try this.......call them on the phone and tell them you have an exhaust 'rattle' somewhere and can't track it down. Out of frustration, you want to replace the exhaust (or at least the cat and muffler, which is all you need to do anyway) and you were wondering how this may affect your warranty. No mention of performance or 'sporty' look. Make sure you get the name of the person you speak to!