The 123GT in full rally trim was as quick as a Cooper 'S' on gravel roads. The rally ones were much lightened with alloy panels, much weight-saving and excellent engines. The overdrive gearbox gave 7 gears and the gearing was just right. They were so strong that you didn't need to slow much for bumps and being RWD it was possible to change gearbox and/or diff during an event, unlike the Mini.
I believe the output from the 'works' rally engines was between 120 & 150 bhp, but with loads of torque.