thing with disc brakes is they lock as efficiantly as a rim brake.. as in, they lock on the disc but there is a small amount of "slip" as with a magura, but because its in the middle of the wheel it is magnified by 13inch (26/2) and the amount the rim slips is more than if a magura hs33 for example had slipped..
prolly doesnt make much sense, and yeh some discs work ok on the rear but i dont think you'll beat a maggie, grind and koxx pads for stopping dead.