I'm very familiar with head buffeting. It really depends on the bike and how it is set up. I had 2008 Suzuki V-Strom that had horrible helmet buffeting and no amount of dinking with different screens would fix it.

Another poster above provided a great description of buffeting, the low pressure area, the turbulence of the air stream as it comes over the screen.

I can absolutely tell the difference between opening and closing the little vent on my 2014 Ultra (aka the redesigned Rushmore bikes). Open, smooth as can be. I have a wind screen I can just see over. Close it, and I can feel my head wobble and the noice goes up.

Helmet shape and style can make a big difference too.

 

Strange, I had an 07 WeeStrom and never had any buffeting, but with my FLHS, and FLHRs, fangs were required to make life okay. I sold my VStrom after getting three speeding tickets in one summer. I could not tell the difference between 55 and 80 when riding. Well, at least that is what I told the officer as he wrote another ticket.