Thanks for the replies and interest. Like I said, AP racing (based in Coventry) will need you to take your car in with at least 1 stock 17" wheel (even if its the spare) so that they can measure up using it. They will need the car for a few hours only.
They will probably use a 330mm disc with the 6-pot calliper for a front conversion. They might be able to do a calliper conversion only for the rear but my experience is that its not worth the money and you'll get the same results by using uprated pads. Even with EBD and brake-assist, most of the brake bias is to the front so the rears don't really do as much work.
Their workshop is open Mon-Thurs I think but I will check this with their R&D dept and post the reply. AP kits typically cost around GBP 1400 but we can of course try to negotiate a discount for bulk buying / offering a car for them to measure up. I've driven several cars with AP conversions and I'm sure anyone who gets one will not be disappointed. Funnily enough, the majority of people in Asia / Australia who buy Impreza STI and Lancer EVO cars change their Brembo brakes for AP ones straight after collecting their cars, which I guess shows the capability and reliability.