I've come to believe that buffeting is more the norm unless you are willing to ride a full fairing bike or install a batwing. I have the Roadster screen and I get buffetting that starts around 60 mph.
The screen on the Bonneville I had a few years back had zero buffeting, but was a small screen, i.e., it just reduced the wind on the core of your body. This was fine with me, but I don't have the bike anymore, don't know the manufacturer and when I go to the Bonne forum for suggestions, it seems they struggle with this as well.
My son has a FJR and he struggles with buffeting...
National Cycle now makes a Switchblade adapter for the Tbird and has screens of all sized for it. I'm hopeful that one might give me the experience I had with my Bonne. I'm thinking their smaller Deflector screen might work.