"decat" as people like to call it, is all very well, but with more and more around I suspect this will be a future target by police. I would get a sports cat if I could, but some Clios seem to have their cats attatched to the manifold so would need manifold modifitcation to change the cat.
I wouldn't get a cat bypass pipe if I were you, but many people do, and haven't had problems with MOT or police. Just be aware is my comment, many people (friends and aquaintences of mine) have been pulled over by the police for a side of the road MOT, you will fail that without a CAT. If you think a new MOT is all it will cost you think again, you're not allowed to drive the vehicle away so you have to PAY for the car to be towed by the police (they'll make it cost about £150) and then arrange for it to be fixed and MOTed again.
I know out of everyone on Cliosport this may happen to a few, but if you play the lottery then you most certainly believe in odds of things happening that might not
BTW ^^That's the only reason I'm not running my car on red diesel as I have a ready supply to tankers of the stuff at work.