i havent done it but i know that without your cat you will fail the MOT on the emissions section. So if you do go decat then you will need to be able to put your cat back on when it comes to your MOT mate. Not sure about benefits though, but there must be some or people wouldnt do it because of the hassle.
You have already said you have got a cat back system, to be fair I would just stick with that as the few bhp you will gain from de-cating is not worth the hassel IMO. Especially since police can now do roadside emissions checks.