Not worth taking the cat off if the car is still standard (no forced induction). Also you'll have the engine management light thrown on from time to time, if not all the time.
I find the camcon to be a good bit of kit as it smooths out the power and torque curves also adds more bhp. Not sure how much they fare on standard cars. there is a thread and I think a dyno chart of a guy who had one fitted to his standard IS by TDi. There's also a guy on here from Italy I think who fitted a camcon himself. There are the diagrams etc on here that he posted too.