I can only speak on the small size shields. Angry fly screen size, Sport screen etc.
They don't really do a hell of a lot when sitting straight up for me, at 6'2" my head is above the pocket. That said, at any speed no matter how high, I can tuck down on the bike and slip into the pocket easily if the wind gets too strong, ya know like when riding say 50 mph into a 100 mph headwind....
I have seen quite a few threads on people trying to solve the wind in the face issues, usually they go something like:
Tall forward screens do block some air but they are too far away and rider still gets buffeting, owners tries every screen they can find and ends up modifying one to work or buys a batwing for the calm pocket effect.