I put a Give air flow 330 and Nippy Normans winglets on and it transformed the ride as you can adjust the height to suit you, in the mountains right down for maximum vision, and on the motorways I have mine half way up and wind free ride. 
I had exactly the same issue with the headache inducing head buffeting - I fitted tobinators to the standard screen, have them set about halfway and have the screen laid back (down) as far as it'll go. The wind hits me in the upper part of the chest which I don't mind and the head buffeting is zero. Granted the wind noise is greater but I ride with earplugs anyway, my entire head is exposed to a constant flow of air so no vibrations/buffeting at all. I came to the GS from sports tourers and semi-naked bikes and the GS screen, even in the lowest position, still provides more protection than any sports tourer or semi-naked.
Also have the Givi Airflow for the winter months for a bit more protection, set up the same as above.
Thanks all for their responses. But I would like to go back to my original question: In what position do you keep your original BMW windshield? This may not be the most important question in the world of motorcycles but I find it interesting since a long fellow like me has two choices: First, to keep the windshield very low when the head is above turbulent airflow and second, to keep the windshield upright and try to lead the airflow over the helmet.
Sorry for my bad english but what is a tobinator?