In addition to carsound, random technologies I believe also makes a cat with a 2.5 inch receiving end. Either way, you can still use you factory cat, you just need to buy a 2.5 inch collector flange from like inlinefour.com or something. If you're in a state that does emissions, you'll need a cat, otherwise just get a test pipe. The stock cat, even the JDM cat is very restrictive, that's the only reservation I'd have about not getting aftermarket ones.

Good luck,
Aj