Hey Andrew and Tamra, thank you for feedback and support. My 818 stops well, and certainly if I stomp the brakes to haul up quickly - it will certainly lockup and R888's will slide (with ABS off). However I still want better braking - better pedal feel, and if I modulate the pedal I want it to stop so that it feels like I'm stripping the tarmac off the road - not just shreading/skidding tyres.
So completed today - I have fitted 2 rebuilt 2 pot Legacy GT front callipers with new 294mm rotors. Ive also now finished installing my 2008 (4 pot WRX) front rotors to the rear along with DBA4657S rear rotors. These are slotted (couldn't get standard) with 170mm handbrake and 316mm diameter. Ive also fitted new HAWK Street/Race pads to front and rear. Finally I also fitted a new 15/16 master cylinder.
Although still running all the setup in, it immediately feels great. Pedal moves a little easier - not as a boosted WRX - but has good gradual uptake for nice street driving - then with a little push brakes bite hard and pull evenly.
Previously on other brake discussions/forums I've liked the concept of matching front and rear brake size and force. Similar to many other mid-engine or rear engine sports cars which have identical front and rear brakes. I now have (from Bob's calculations above) 3.37" brake calliper (force/area) front and 3.082" rear - with no brake bias.