Hi Stu,
Assuming you've gone for the CP9200 caliper with 330mm disc (not the larger 355mm setup). The best and cheapest option is just to keep the standard rear calipers & paint them. The rears don't really do a lot or generate anywhere near the amount of heat that the fronts do, therefore making a rear BBK pointless (looks aside).
If you buy AP's rear caliper setup, I expect it will substantially change the brake bias with the CP9200 setup. On this note the AP Rear disc's are actually bigger than the front discs at 335mm. It will however work with the larger 6 piston 355mm setup.
You could also do what I've done, which is get a set of the STi rear Brembo calipers, this has a slight affect on brake bias but it's hardly noticeable. More information can be found on my build thread:
https://www.gt86.org.uk/forums/forums/topic/4097-will300s-subaru-brz-build/?do=findComment&comment=105056