If you search, you'll find some threads on modifications and warranty. Long story short, if you replace an OEM part with an aftermarket part like an exhaust, your warranty wouldn't apply as to that part. But if there is a failure, the dealer would be obligated to do repair work under warranty unless the aftermarket part caused the failure. That is per the Magnuson-Moss Warranty Act. Here is a useful link:
http://www.dummies.com/how-to/conten...ty-intact.html