I wonder how Moss and Jenkinson managed to average almost 100mph around 1000 miles of 1955 Italian roads in a car with "terrible" brakes? The 300SLR won every race it entered, apart from le Mans, when the team was withdrawn after the crash.
The brakes were not terrible at all, but they weren't quite up to the Jaguar's Dunlop disks, and the airbrake helped level the playing field, particularly after the long Mulsanne straight.
Interestingly, Moss used the airbrake also as a steadying aero device, though perhaps he was the only one to do so.