If you ask me, BMW eliminated the cooling to reduce air turbulence in the wheel well...an MPG thing. BMW has been big about wheel area turbulence which explains the slots on the sides of the 1M bumper.
BMW probably thought better air flow management in the wheel wells is better than the limited effect of the brake ducts.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red