I'd say it's somewhat on the quiet side, about the same as my prodrive axleback, just not as deep. It was one of my favorite exhausts, and I liked how the inlet flange was made. It allowed you to run a downpipe with a flat flange or donut gasket, without leaks. However, it is not a straight through design (baffled I think is the term), but the restriction is minimal and shouldn't make a difference unless you're making 300+ whp.
Btw, a catback shouldn't void your warranty at all unless they prove that it was the cause of a problem.