That Z4 is uber stealth, but personally not a fan of the Hartge wheels at all.
The BMW brakes get more stick than they should, they're going to fade like any other road application with boggo pads and fluid. Worst thing is to leave TC on.
Andy's solution with the BMW Performance calipers is great VFM... only thing is some people in the US have had trouble with them. In an ideal world you'd get to try everything out beforehand, but i might of crossed my fingers and gone with that kit instead of APs if they were around when i bought them.