[Richbot]

I wonder how this effects the aero balance? Ducts are great as long as they don't increase lift. There may be a reason the 335 has ducting and the M3 doesn't...probably wouldn't matter at most tracks though, even if it did increase lift.

One of the reasons a ducted backing plate can be so effective is that it employs the pumping action of the vanes in the rotor to both move more air and fling it outward through the spokes of the wheel, which helps get rid of some of the high pressure air you're introducing to the wheelwell. In this case you're getting the fire hose effect, which has been SOP on most BMW's for a long time (simple hole in the splash guard with a duct leading to the lower edge of the bumper). Has to be better than nothing for brake cooling anyway.