[Cyberdemon]

Don't see why this would be worth a cent of your money to try to retrofit.

The rear brakes do very little work compared to the fronts, and the M already has very capable stoppers, which can be made more capable with a high temp fluid or more aggressive pad.

So other than filling out the wheel well with a mismatched caliper there is zero point to this.

If you are unhappy with your brake performance there are steps you can take, if you're trying to modify a complex piece of engineering based on a good deal from a friends leftover parts...then beware.