I just bought a set of 3" alu flanges and I'm planning on doing the brake duct mod.
I plan to cut & fit them to the underbody panel, then anodize them black and use OEM fasteners to secure them into the panel. I'm thinking of providing the brake ducting just into the wheel well plastic shroud, without going all the way to a backing plate. My reasoning is that my previous BMW's all had this type of arrangement and I was fine at the track when using track pads.
I'll keep everyone posted with my progress...