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I would have though that disc brakes were inherently disadvantaged in one way, in that the leverage you can exert is much smaller because the disc is so much smaller in diameter than the wheel

you think wrong. they are more efficient even in the cable variety. that's why road/cross disks are tiny compared to mtb disks it's easy to overcome the grip of a 23c tyre with a 140mm disk. the hard part is ensuring good modulation.