I’d say wind becomes more of an issue at higher speeds. Once you go over 100mph, the NineT gets uncomfortable, even though the seating position is (usually) leaning forward quite a bit. So, in the US with its rigorous limits, and cops all over the place, I personally don’t think it’s an issue and for that reason, a windshield isn’t making a difference, unless you mount a giant “touring” screen, which IMO looks a bit weird on this type of bike. I can sense a drastic difference though when I’m wearing a solid leather jacket as opposed to a summer perforated one, the solid leather works almost like a sail, again, at higher speeds. In Germany, at usual continuous travel speeds in excess of 120 mph, I definitely wouldn’t want to be without one;-)
And over 150 mph, even with a fancy race fairing, you need to buy a very suitable aerodynamic helmet, to avoid your eyeballs shaken out of your skull...