Has anyone left the Cat on after SVA to use as an extra silencer, or does it not make that much difference? I was thinking of the 200cpi variety.

I left it on as cat.
Saves fines from the increasing number of on the spot emissions tests they seem to like around where i live.

Does it reduce the sound by much then?

That's the point, if it acts as a silencer I think I will keep it on, would be another job not to do after IVA.
I have fitted down pipes similar to bike with pipes running down and out behind gearbox plenty of room for cat inside bodywork if I keep it on, otherwise fit it outside and take off later