In "theory" braking ability should have nothing to do with tires. A good driver should be able to modulate the pedal well enough to invoke maximum stopping ability without locking up the brakes. This is of course on optimum surfaces in optimal conditions.
Realize that not ALL stopping conditions result in the necessity to lock brakes. Better brakes will not fade nearly as quickly, not warp as often and stop under much better conditions.
fyi, the rears only control 30% of your braking, and a switch to rear discs in just the rear by itself will not note any significant improvements by itself without adressing the fronts too.