A cat is not fitted to an engine to improve it's performance, therefore why retain it if you don't have to? The cats have been off of my '91 MkIV for the last 10,000 miles. The engine breathes a little better and there is a small improvement in performance. There is other stuff on the car, such as the air pump, that can be removed if your local car emission laws allow. The standard engine powers a pump to add fresh air into the exhaust in order to help meet emissions regulations. Of course no one mentions that the engine consumes a little more fuel to power the pump in the first place. I have also removed the small back boxes from the exhaust system, I now get about 24 mpg from the engine and it's not so asthmatic.