[dc_wright]

The 3.0si gets a bit of an upgrade in the calipers from the other non-M Z4s and uses the same front calipers as the E46 330i, and the X3, though not quite as robust as what the M cars get. With a good set of rotors and pads they're fine for autocross and adequate for short session (20-25 minutes) track days.
The cast iron BMW uses for the caliper housings does develop a pretty nasty looking patina, but they clean up really well with a wire wheel or a wire cup brush and a coat of G2 caliper paint will keep them looking good. Here's before and after on my front calipers.