Apparently Bendix did a study on discs vs drums on semi's. Not directly applicable (best I could google on short notice) but perhaps something can be taken from it:
A study comparing the braking effectiveness of air disc brakes versus air drum brakes for tractor-trailers was conducted by Bendix Commercial Vehicle Systems. Tested at 60 mph, the disc brake stopped the vehicle at about 200 feet and the drum brake did so at about 270 feet. This braking distance difference is the result of the different braking systems. After 15 stops from 60 mph, the stopping distance for the drum brake deteriorated from 270 feet to 450 feet, while stopping distance for the disc brake remained at close to the initial 200 feet.
The increased braking distance for drum brakes at successive stops is the result of brake fade.