De - Catting the car should make a difference, in my experience a NA car still gets some benefit, although not necessarily all out bhp but in the responsiveness of the revs and driveability of the car.
The cat is there to help reduce emissions. If your car was first registered 1993 or after (unless an it's an import without a uk equivalent like an MR2 Turbo) they will do a "cat test" at the station. This is a lower emissions standard than the earlier cars. If your car is tuned well it can pass without the cat however, the cat sensor (lambda) is a part of the self measuring process in the car, it feeds info to the ecu. I'm not sure if there is a decat with a hole for this sensor about.....
The old bill won't know whats going on if you car is not blowing blue smoke but the random VOSA checks which are often set up these days will because they will test it and impound it at the roadside if it fails.
So it's up to you, but every time I think about it, I can't be arsed.
