Recently purchased a lovely red 06 vrs which has been mapped by jabba sport I'm looking for a bit more grunt would a sports cat/downpipe make a noticeable difference ?

Dunno about noticable, but note that if you go TOO big it's likely have a negative affect on the performance as the gases actually whirl around inside the unnecessary space in the pipe instead of out the back as they should and affecting the natural optimum flow. Completely the opposite of if you have a tiny bore which chokes the engine cos it can't get the gases out, this time they can't get out cos they're kind of milling about instead of dynamically exiting.

Yes it does , turbo spools up quicker , but you will have to get the car remapped to make the most of it. Usually a performance air filter or a recommended induction kit will also help. Yes you can go too big , but most downpipes and cats for the golf mkv gti will be o.k. This has been done to death on this and golf mkv gti forums. This is usually refered to as stage 2 and the next level is stage 2+ which usually involves the addition of an uprated fuel pump. I'm sure jabba sport will do a minimal charge for stage 2 remap. Give them a call!