Originally Posted By: armos
The catalyst parts will end up inside the muffler eventually, where they will rattle forever. If you're planning to replace the whole exhaust anyway this might not matter though.
There is a resonator after the cat and before the Y pipe splitting to dual mufflers, so it would probably stop at the resonator, which needs to be replaced as well. However, for the resonator to be replaced, the old one needs to be cut out and a new one welded in, but I would probably just opt for a full cat-back system.