If you fit the blueflame Dp and cat to a milltek cat-back system you'll need to ask blueflame for a reducer as the internal bore of the milltek is smaller than the blueflame.
There's a pic somewhere on the forum showing a side by side pic of the standard cat against a milltek and blueflame and the blueflame one is wider throughout (2.5" at it's narrowest IIRC and 3" throughout the rest) whereas the milltek one doesn't look much different to standard as far as bore size goes.
I got quoted £330 for a blueflame race cat last month but I've heard members paying £318 normally so I think that's more like the going rate.
I've got the Milltek cat-back system myself and am definitely going for a blueflame race cat, just hoping the customer service isn't as bad as I've heard.