I am running the 18/17 CCW set up with 356mm 8 pot K-sport front brakes and J-Spec rears. The car had 4 pot K-Sport rear brakes fitted by the previous owner but they had been shaved to fit behind the 17" rear wheel and were still very close so I decided to remove them and fit J-Specs.
I am soon to look into fitting UK rear brakes just to see if I can get away with it. The J-Spec rears are not to bad with the right pads in but under very hard braking for long periods they do fade, as found out on DragonBall 2013!
In my opinion the 18/17 set up allowing for a larger rear tyre side wall is a great aid to traction. My car grips like hell and doesn't seem to loose any more traction than my old BPU Supra with over half the power. My only issue is tyre selection as although the R888's are great in the dry, they will try and kill you in even straight line cruising when wet! I wish I could get hold of some better all round tyres for the rear.