Alot of it really depends on your relationship with your local dealership.

Also the MM act regulates and explains aftermarket parts and warranty issues.

As stated above unrelated items will still have a warranty. Simply changing the something like the cat-back will not void anything more than the warranty on the parts you removed unless you sneak them back on before taking it in for service.

Stuff like heads, cams and complete intakes will however kill your engine/powertrain warranty.