Buy a high flow cat. I recommend a carsound / magnaflow cat. They flow better than pretty much any other cat on the market.They're universal but pretty much any muffler shop will weld the flanges on.

You should probably get at least 2.25 inch piping from the cat back if not 60mm (2.38 inch) or 2.5. The bigger the diameter, the more air flow. Whatever you do, don't get a cat that's bigger than the exhaust piping (say a 2.5 inch cat to 2.25 inch pipe). You want the diameter to go from small to big OR stay the same the whole way (2.5 collector / cat / exhaust)

What i did was run an aftermarket header (comptech) with a 2 inch collector (stock size), to a 2.25 inch carsound cat, to 60mm Tanabe cat back. So the diameter is increasing the whole way out.