Here is where my quest for smoother airflow has lead me. Each iteration seems to have progressively gotten smoother for my head. Starting with me, I am 6’1” tall with a 35” inseam which makes me very long legged for my height. I am 185 pounds and am using the Suzuki tall seat.
First was the Givi windscreen, much better than stock, using the mirror extenders I cut down the head buffeting significantly but there was room for improvement. I ran the windscreen in the lowest bolt holes and the sliding portion almost at the lowest position.
Next was the eBay special cheap aftermarket mirrors in an effort to cut down on the turbulence from the big stock mirrors that was hitting me on the sides of the helmet. The mirror change yielded less buffeting but the buffeting vibration was at a higher frequency. At 60 mph it was fine, at 80 mph, it was a bit uncomfortable. Here too I ended up running the screen in the lowest holes and the sliding portion all the way down or up an inch or so.
To try to combat this higher frequency buffeting I found that a couple of rubber washers under the lower screen bolts to tilt the screen back helped along with running the screen in the highest bolt holes with the sliding portion all of the way down. The high frequency buffeting was definitely better but was still there at higher speeds. Additionally I did not like the screen up high as sometimes I ended up looking through rather than over. The other thing that I noticed with running the screen higher is that while it helped with the helmet buffeting but it left more wind blast on my shoulders than I cared.
I got the idea of trying some fairing spoiler pieces after I saw that people were having good luck with them on the prior generation bike and thought it would help with the shoulder wind blast that i was now getting. One of the things I did notice with the higher screen was more noise through the bottom of the helmet which was bothersome even with ear plugs.
I bought a plastic trash can to cut up to try out the fairing spoiler idea. This was a definite improvement with the shoulder wind blast and the wind noise was definitely reduced. The side effect though was more of the high frequency buffeting from the small mirrors was now hitting my helmet. I then lowered the windscreen to the lowest bolt holes (still with the rubber washer spacers to tilt the screen back slightly) which was an improvement to the head and made the airflow over my torso very comfortable.
I started thinking that the replacement mirrors were now part of the problem. I went back to the factory mirrors and the mirror extenders. Wow, that was really an improvement, the high frequency vibrations were gone even at really high speeds. The factory mirrors make quite a bit of turbulence but it is a much lower frequency and now this turbulence is no longer hitting my helmet. The combination of the low screen and the trash can spoilers are pushing the almost all of turbulence outside of the space my helmet occupies when riding.
I think I have hit on the magical combination of things that has lowered the turbulence below the level of many other bikes I have ridden which is a very large improvement from the stock bike. Right now my trash can spoilers are held on by Velcro. My next step is to make some nicer spoilers out of carbon fiber and either stay with the Velcro or use some double sided foam tape to adhere them to the bodywork.
This was my journey, maybe it will provide some ideas for others that are also fighting with buffeting.
Update: I have played with moving my spoilers backwards and forward and I have found a spot where I am totally free of buffeting both with and without the tank bag. My quest to fix the buffeting problem is over. I cleaned up the cut edges with a flame and for the time being, I am going to skip making carbon ones and just ride what I already have.
In summary, Givi Airflow windscreen in the lowest bolt holes, two rubber washers per side under the lower bolts to tilt the screen back slightly. Locktite was used to make sure the bolts did not back out over time. Sliding portion of the screen all of the way down. Stock mirrors with the Adventure Tech mirror extenders. Trash can spoilers as shown in the pictures, Velcro attachment.