Braking performance

[Gnasher]

just MOT'able thickness.

 

What's this then?

Your probably find the reason the old disc weren't performing was due to seized semi floating rivets, by changing the discs for ones which the rivets where not/less, restored the pad contact area and the power.   

My only reasoning is that maybe the callipers work more efficiently if the pistons aren't stuck right out as they compensate for both pad and disk wear.

The length the piston protrudes has no bearing on their performance while in normal use inside the caliper bodies with a pad and disc.  If the pad has no friction surface left, is not square to the disc or held off due to seized rivets/s/warping or the pad is twisted in the caliper body (very common with EBC as they are slightly smaller)  power is greatly reduced.

The thickness of the disc is purely to ensure reduced sensibility to constant heating and cooling which warps them and to ensure longer service life.  The fact these have semi floaters is another way to reduce their thickness, thus unsprung weight, fully floating disc are thinner still.  The brakes fitted to the earlier and later Fazers are eye popping the Fazer is really over braked, but they need carefully looking after to keep that performance.  Brakes are a bit like eyesight they slowly deteriorate and you don't really notice it until you either need them in a hurry or you ride another bike who's brakes are tiptop.