Does the caliper actually pivot on the adapter, or do the bolts hold it tight? The deal on master cylinder piston size is the smaller one gives higher pressure with the same squeeze on the lever, but less volume of fluid per mm of movement compared to a larger diameter. If you want power brakes go with the smallest one that won't bottom out against the grip.
I've been thinking of changing the front brakes on my R90s, back in 1980 there was one racing that used Lockheed calipers on hangers that pivoted on the front axle and had a pushrod that attached to the lower fork crown. This gave an anti-dive effect. Looks pretty cool too. But I like the way you guys have used the Japanese calipers.