No replies so far and I don't have practical experience of actually fitting one!
However, I have looked into this, the Clubsport S back box (£350) sounds good, giving a relatively small improvement.
Not a brand I have any experience with, but a few tuners I have spoken with have recommended Scorpion systems (£700), I heard one in the metal and was impressed.
You could push the boat out and go for an Akrapovich, having heard one on a TCR, I don't feel it was worth the extra expenditure over the others mentioned?
As and when / if I change the exhaust on my CS, I would probably go for the Scorpion non res.