I've always had the flat thumb, seems like more contact area = more spin, power and speed. One of many reasons why bigger hands lead to more spin and speed, it's just more contact space, same reason why a larger brake pad would add more torque and power to stop a car. Bigger brake pad = better brakes, larger hand = more contact space on disc = more speed and spin. I'd argue a flat thumb is an easy way to increase thumb size and contact on disc.
I know from my indoor rock climbing days that the larger grips and handles on the wall automatically make grips stronger, for the simple reason of more contact space on grip. Seems like this an appropriate comparison for gripping discs.