In theory, discs should be better. But the brakes on mine are superb with drums.

I can only suggest that it is due to the fact that discs are too much for the rear of an already light car.

The only real way to improve this is to decrease the rear bias, in which case you might aswell have kept the drums.

I haven't driven many RST's with rear discs, but those that I have driven have had shite brakes.