The car already has a code red map on it so will be fine with the bigger downpipe. The map may also have the parameters of the post cat sensor already widened so not to bring up faults when using a sports cat. I'd imagine this would be easy to find out by just giving code red a call or email. My bluefin already had the parameters of the post cat sensor widened, so when I added the downpipe and sports cat there were no issues.
If you are still thinking about a mongoose system, there is a used cat back system on ebay currently at £175! Also in the classified section on www.focusstoc.com there is a cobra 200 cell cat for £150! That would just leave you needing a downpipe and these regularly pop up in the classified sections and on ebay. I recon you could get yourself a full turbo back system with sports cat for somewhere on the region of £500 - £600 which would save you an absolute fortune over buying a new system.