Are you talking about helmet buffeting or bike buffeting?
The cleaner the air flow- in other words, the farther your head is away from the aerodynamic changes of the windscreen, the less the buffeting on your helmet. Think of this as if you had no screen in place and then your helmet is in "clean" air.
OR
If the screen is big, you can be in the bubble behind it and not get any buffeting on your helmet, but the bigger the screen, generally, the more you'll feel buffeting of the bike itself and that can feel like the engine is missing.
So there is a balance of height vs protection from the elements to be achieved and that is different for everyone it seems.
The worst combination is to have a screen that is large enough to allow buffeting of the bike, and to put your helmet in turbulent air flowing over the screen so that helmet buffeting also occurs.
Trial and error, then compromise it seems is the rule. Unfortunately it's an expensive trial and error, unless you can find a shop that has a selection of a number of different screens and you can test ride them. That's rare, I think.
I personally think aesthetics are also seriously important--I'm very superficial that way. I'd suggest you look around here and other BMW fora, look at the screens you like the looks of, contact the people who have them and ask specifically about buffeting and protection.
Then you pays your money and you takes your chances.
ymmv, I spose, but there are only a few people who have experience with more than one or two screens, and their impressions are dependent on how they ride, how they slouch, torso length, where they ride, etc, so many variables that you could be at square one and not know it, but think you're getting worthwhile insight. Screens are a personal thing.
