Junk the sports cat and go full decat.
I felt the restriction of a sports cat when I was running 300BHP, so you will defo get a benefit running the 20G.
May also need to consider a full 3" system rather then the 2.5".
The revolution 3" is supposed to be quite good and not too loud.
Then of course, speak to CJ and get him to cut your bumper about and fit a FMIC (he's done a few now).
£540 for Revo Downpipe and System (3")
http://www.revolution247.com/erol.html#69461X0
£185 for TSL Decat Downpipe (fits to std 2.5inch system) :
http://www.tsl-motorsport.co.uk/newsite ... &detail=38