Here is quote of guy(John Felstead) who tested all of them
With regards to the K sports, you may have 8 pots, but the surface area of those 8 pots is almost identical to the WRX 4 pot and less than OEM Brembo's. The K sports have two small leading pistons working on the area of the pad that provides the main performance of the pad, which will give less bite from the pad than the larger leading 4 pot piston. You will get a better performance from a 6 pot caliper with properly stepped differential pistons. The equivalent D2 6 pot caliper has larger piston surface area and a larger leading piston than the 8 pot K sport, so gives better initial bite and more brake force for the same pedal pressure.
AP 6 pots have better differential piston sizing than both the K sport and D2, with 3 piston sizes in use to ensure the pressure on the pad is well matched to the requirement to provide less taper wear and give a better match to the required pressure increase as you move away from the leading edge of the pad, which is where all the work occurs. The D2 6 pot has 2 small and 4 large pistons per caliper, so the pressure required is less well distributed than the AP, but not too far away, the K sport has 4 small and 4 large pistons per caliper, which gives a less even pressure match across the pad length. This means the AP will give the better brake feel and modulation, with the D2 6 pot next, the K sport a bit further away.
With regards to piston surface area, pad surface and the total brake torque resulting from that, on a 330mm disk the D2 6 pot gives the most total brake torque, K sport next and then the AP 6 pot last. None of these give as much brake torque as a Brembo OEM caliper on a stock newage disk. Even a 356mm AP 6 pot kit gives less brake torque than an OEM Brembo setup. Where the AP, D2 and K sport win is in handling the elevated temperatures seen on track.
There are differences throughout the Subaru range with regards to the master cylinder bore size, and also pedal ratio, this will also influence the amount of force required at the pedal to give the required brake force. So a brake kit on a classic will work differently on a Newage STi, Newage WRX or Legacy. And we haven't touched on brake balance matching for the rears, or the way the ABS system will work when you change that balance.
