Well, a test pipe is definitley better flowing than a stock cat, but so is a high flow cat, and a high flow cat is legal. Like import810 said, you'll repay the money you save by getting a test pipe instead of a cat 10X over if you get caught with one. And how do you plan on passing emissions at inspection time?
A test pipe makes your exhaust stink like holy hell.
Don't need to worry about doing an O2 sensor bypass if you get a high flow cat.
Don't you care about the planet, the air you and everyone else breathes? Get a cat!