If you can get it low enough it won't hit your helmet at all, but the air hitting your chest may go under the helmet and try to lift it up some. Mine did that with my full face, but if I left the shield up a click it improved it.
I couldn't get my tour version high enough to stop all buffeting, but there's an easy mod for it.
Remove the shield leaving the brackets in place. Measure how far the lower bracket is from the headlight. Loosen the lower brackets and rotate them outward about 1". Tighten them back down, then bend the bracket back in towards the headlight to the amount you measured before. Your goal is to position the mounting screw on the bracket right over the headlight nacelle mounting bolt.
You can play with the position, but what this does is lower the front of the windshield and elevate the rear. You can lift the rear of it up at least 4" more doing it like this than you can by just raising and lowering the shield on the forks.