Looking at the brakes on the CS-S, the rears appeared to be ventilated and the same as the GTi PP upgrade, which I assume is the same as the R?
The only difference was the fronts, the caliper looked the same size as the PP, but the disc had a separate bell, rather than the regular solid vented disc and the obligatory "cooling" holes drilled. Both concessions to cooling and help toward the track intentions of the car.
Maybe it was a step too fat to fit a cage in 400 examples of the CS-S, in that the TUV paperwork would have been excessive.
Even though the cage would have been one of those creaky, half hearted Meccano bolt in affairs, it would have provided a solid structure to attach a 6 point harness too.