Great topic Troy. A lot of good input from fellow members. Here's my $0.02:
First of all, we can all agree that front discs are far superior to front drums, that's a given.
Braking tranfers weight to the front tires, the harder you stop, the greater the weight transfer. Rear engine cars have an inherent braking advantage since the heavy lump of metal is not already loading the front end, but make no mistake, significant weight is transfering forward even with a rear engine. The rear brakes have an interesting job. At slight braking 60%+ of the weight of the car is on the rear axle. In a hard emergency stop, weight transfer may only leave 30% of the weight of the car on the rear axle. The worst case is having the rear brakes lock up prematurely in an emergency stop due to lighter loading causing a potentially disastrous spin. (Ever been on ice and have the back of the car suddenly pass the front of the car, giving you a great view of the road behind you through the windshield? Congratulations if you survived that experience). To prevent his scenario, engineers use smaller less powerful brakes on the rear axle, smaller diameter rotors, or drums, proportioning valves, anti-lock, etc.. Look at a modern car, you may have 13" rotors in front with 4 piston calipers, and 11" rotors in back with single piston calipers.
A locked wheel, whether with or discs, has the same stopping power. The difference is the additional sensitivity and consistency of a disc brake vs. a drum brake that may be more prone to premature lock. My experience has shown me that at the limit, I can better trust discs, where drums have often surprised me, when driving at 10/10ths. We all know that maximum braking happens just on the verge of lock up, skidding to a stop greatly increases braking distance, not to mention the risk of loss of control if rear brakes lock up before the fronts.
So if you slam on the brakes and slide to a longish stop, it does not matter if you have rear drums or discs, at that point you are just along for the ride. If your rear drums are adjusted perfectly to lock up a millisecond after the front discs and you can consistently apply the perfect brake pressure to stay right the the edge of lock up until the cars stops, then again no difference, stopping distance will be determined by the available traction between the tires and the road surface.
But in reality, no one can consistently and continually adjust rear drums so finely, and unless one of us in currently a factory formula 1 driver, our braking foot is also not so finely calibrated and skilled. Without modern electronic wizardry, rear discs offer much greater sensitivity and consistency when driving at the limit. Now for street use, if anyone is driving that hard on public roads, I pray you are not anywhere near my family risking the lives of my loved ones. I don't drive hard enough on the street to notice a difference, I save that for the track.