Yes, as all the above have said, a decat will remove just the second (main) cat. There is a cat in in the manifold too, some people have removed this also by changing the manifold. You should still pass an MOT unless the tester realises that the car came with 2 cats (unlikely).
Unfortunately I have to change my downpipe and decat back each year for the MOT as with it removed i have a straight pipe from the turbo to the backbox, but it's a fairly quick and easy job.
I wouldn't bother personally, I did it on mine as it could be as much as 15hp difference for me, but on your car you're probably looking at 1 - 2hp at most.
A sports cat will give even less of a performance increase so unless you have an issue with your current cat, leave it as it is.