I spoke to Wullie @ Meercat today when i was getting a heat shield made up for my RCM induction kit.
He says SNAKEHEAD DOWNPIPE WITH 100 CELL CAT IS £430 FITTED to a 3inch or 2 1/2 system or you could get a SNAKEHEAD DOWNPIPE AND A 100 CELL CAT IN THE FRONT PIPE to a 3inch or 2 1/2inch system.
Personally i would go for the 2nd option as then the snakehead will have a better flow.