The M50d comes with these large calipers, the rotors are 385mm, you could buy these rotors & calipers off BMW but they require 20" wheels and the dish shape of the wheel will have to suit also as there is not a lot of clearence.

Your best bet would be what I did on my old M-Sport 35d, is have the calipers painted and install ceramic pads, either from Carbotech Brakes in the US or EBC in the US or from the UK, they have a new version of there RedStuff pads which are ceramic.

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X5 M50d White | 20" Rims | Adaptive Drive, Self-Lvl Susp | Act/Steer | HUD | Xenon, Adapt Lights | H-Beam Ast | Rear/Side Camera | Panorama S/roof | Auto Tailgate | Comfort, Active, Ventilated, Heating, Lumbar, Navada Perforated Leather Seats | Comfort Access | M-Steer Wheel w Pdle Shift |Sports 8-Speed | Active Crus Stop/Go | Proff Nav + TV, DAB | 16 Spk Ind Audio | USB |Internet-eMail-SMS-Apps | 4 Zone Air | Speed D/play | Towbar | 20% Tint | 5mm Castor Bushes | Yellow Calipers, Ceramic Pads |