Once the pads & rotors are properly bedded in, it will stop even better. The smaller bore coupe rear wheel cylinders actually reduce the braking effort of the back brakes. This is because the discs are a non servo brake type [they don't assist themselves to apply, like the drum brakes did] so more line pressure is needed to apply them.

This extra pressure was leading to undesirable rear wheel lockup under some conditions, like in the wet, in 1200 coupes, which were the first of the small Datsuns in Australia to get discs.
Under ideal conditions, the front wheels will lock just before the rears. Well, thats the plan.