Hi Alistair,
The brake dust you get is from the pads, rather than the discs - as you say, the discs on 924s can last for a very long time. The 'bedding in' part of the EBC pads (red surface) treats the surface of the disc, putting the steel through a heat treatment cycle and adding an alloying material to it. This sinks about 1mm or so into the disc, and happens each time you use an EBC pad. The benefit is reduced fade with EBCs since they've had this coating, and pretty instant braking performance. The problem is that when you use a different pad it doesn't suit the hardened disc and with something like a CL (which are very coarse), you rip the surface of the disc.
Anyways, all 924 racers should change their discs as soon as they see surface cracks appearing - it's the cheapest thing that stop's you hitting something hard.