On both GTD's I've owned the rear brakes have been in a worse state than the front - at relatively low mileage too. Its a common observation on forums - explanation seems to be, as above, the stability system uses the rear brakes a lot to improve cornering and the ACC system uses the rear brakes to adjust speed. Apparently this is to stop a 'pitching' motion that would be apparent if the system was using the front brakes. So unlike older cars, and totally unbeknown to you, the rear brakes actually work quite hard.
Its not too bad a job to do yourself if you're handy with the spanners. Plenty of guides out there.
