If I were you I would get a high performace cat from random technologies. it won't really cost you any performace. If you do take the o2 sensor out the computer will give you all sorts of trouble. And if you just get a test pipe you will have a check engine light for the rest of the life of the car. The computer expects 2 different signals from each of the sensors. otherwise it freaks out. So just use a high performance cat.
but that is just what I would do.