Regardless of whether or not you have emissions testing in your state, if your car came originally equipped with a catalytic converter, federal law states that you cannot remove it. There is no mileage clause. You must replace it.
This is true in Canada as well, and many other countries. (Bangladesh, UK, Netherlands, Thailand, etc.) The wording and laws slightly differ, but the point is, if you modify or remove your cat, you could be in big trouble. And there are alot of exhaust shops that either won't work on your car, or will turn you in.
If you get pulled over, and a Law Enforcement Officer smells or hears what may be the removal of a cat, all he/she has to do is look under your car. A high-flow unit will provide the backpressure you need, as well as keep you legal, and keep the environment cleaner.