Screw the car. It's totally messed up to remove the cat because of the emissions. Everyone would have to breathe the air you pollute, and that isn't fair.

If you want more performance, get a high-flow cat. A good high-flow cat will flow better than your MUFFLER. If you don't have the money for a high-flow cat (Like $100+labor for a custom one and $200 for a bolt-on), then don't mess with it.

Anyways, you'll only notice something swapping ths part out if your cat's getting clogged. Otherwise, the simple advice is DON'T REMOVE YOUR CAT.

Again, It's really bad for the environment.