I think you have hit the nail on the head. It would be not only the best way but the cheapest way to just have replacement calipers available for the M3 to bolt start on.
Has anyone contacted ATE, are they not the suppliers of the brakes for most BMW and would they have another caliper to put on or is that imfomation incorrect?
X5 F15 M50d White, 20"rims, Yellow Calipers, Ceramic Pads. Better off saying what it does not have and that's a B&O Audio System, otherwise all option boxes are ticked.