I've left the Manifold well alone too mate.
I'm using a K-Tec 200 Cell Sports-Cat and a K-Tec (Magnex) Cat-Back System. This is a 2.5" I.D system with 3" slash cut tailpipes that fit nicely in the holes on the diffuser, it actually looks standard which I really like. I think K-Tec now have their systems made by somebody else other than Magnex, I've heard it's JP Exhausts (very good company, they've done a system for me in the past) but don't quote me on that.
There's definately been an increase in performance with this setup, there seems to be a bit more oomph in the mid-range and the kangarooing seems to be less obvious also, strangely. It is loud, but it's not over the top. I can't comment on the non-Magnex K-Tec systems though as I've not heard one in the flesh before.