The old rule of thumb was that rear discs didn't improve braking until you were stopping from 150 mph or higher. Drum brakes are really good at slower speeds but they heat up fairly quick. Disc brakes run cooler so they'll do a better job at a high speed stop, or repeated stops. On a drag car the weight advantage of disc brakes, and the easier maintenance can tip the balance. Dual calipers on the rear can help holding a car on the line.

Whatever you do, you have to make sure that the piston sizes are balanced front to rear with the master cylinder. It is a hydraulic system and the caliper pistons will move according to piston ratios. If you mess up the balance between front and rear piston size then the car will never stop very well.