Roll rate is dependent on wing area/aileron/area, among other things. I see your point, as the area of the wing was not "halved" as was the aileron area. Good point.
Having a shorter wingspan reduces the inertia of the rolling moment? Less energy is needed to start, maintain roll? The aircraft is also less stable in roll.
The rapidity of the wing drop is what captivates me, not the impulse strength of the ailerons. I doubt the snap left was controls induced, more like a wing drop from loss of net lift v/v the right wing? Wake turbulence remains a possibility? Torque from the loss of structural integrity at the Tail?
Clipped wings, quick turns, eh? I am waiting for the next gen Mustang to show up with anhedral?
As to airfoil, the aileron is just a wing with a humongous cuff. And the laminar flow Mustang is sensitive to interruptions in local airflow?