Well it depends, other than voiding your waranty, are you planning any other exhaust modifications??
Anyone correct me if I am wrong, but the O2 sensor requires a certain ammount of back pressure to opperate corectly, also if we have more than one O2 sensor ( example one before and after the cat) one will get a bad reading and potentially throw a code) which could have detrimental effects on performance, unless there is a way to give the computer a constant good on the one after the cat (if it even applies to us, I have not had a chance to go under the car) IMO especially if you have any kind of state inspection i would leave the cat and upgrade everything else (headers and 2.5+ cat back) and see comprobale gains, like Vita said, we already have a "high flow"