It's all personal anyway: what works for one, is a headache for somebody else. At 6"5 the OE screen on my 850 works perfectly fine, speeds over 100 mph are no problem even for longer. However, when I duck my head it gets noisier so shorter people may be less impressed. Again, if higher screens cause buffeting, don't go even higher but try the opposite. You get used to a bit more pressure soon enough, but buffeting and noise will always annoy.
Leaving a slit under the screen can surely help releasing the vacuum on higher screens. However, I remember riding a Pan European in the rain, drops were gathering around the screen's lower openings and when they were big enough, flew right into my face: very unpleasant. It worked better on my old BMW R1100RT: The great thing of that bike is that its screen can be adjusted by a push on the button, therefore revealing (for me at last) a lower setting was far more pleasant than higher ones.
