In BAW 5390, GBJRT in 1990, at approx FL170, not only did the windscreen get projected out of the fuselage, but despite the pilot having his lap belt on, he followed it nearly all the way, so I would suggest pressure differential and exiting airflow is stronger than headwind component. Now that was due to incorrect size bolts being used to fix screen to mounts, so possibly not the same as if the screens managed to delaminate/shatter but mounting frame still securely attached...