I don't see the point. You could probably get a 4 wheel BBK cheaper than trying to buy the M5 brakes. I would imagine that they would be hard to find used.
Plus, the rears are still 1 piston sliders.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red