De-cat will mean that you will have to have a cat put back in for mot, and taken off after wards. unless you have a friendly mot station. And if you cant do this, it will cost money each year to swap over.
Other option is a sports cat, cost about £200 and are a LEGAL high flor cat, almost as derestricting as a de-cat but will still pas an mot.
Other thing is that traffic police DO carry emmisions testers to do by the side of the road, and can find out if you have no cat on your car. Which COULD get you in big style trouble. That isnt worth it. But that depends on how you drive to draw attention to yourself.
Lots of points to ponder there...