ph001 said:
The backing plate is designed such that it allows a hose to be attached that is connected from some entry on the front bumper which vents cool air directly to the centre of the disc. The centrifugal vanes of the disc pull this air through the disc allowing much much better management of disc temps. My discs on track are getting to around 600c which is a little on the high side and whilst removing the oem ones completely may help a little this would expose the ball joints to these temperatures directly. The standard Z4m air 'ducts' in the fangs do bugger all for getting cold air even into the wheel well.
There is a great write up on here by exdos with another perhaps slightly cruder (in terms of the backing plate) diy for a similar idea.