Well, this is easy to explain. In stead of big formulas, here is a small schematic representing the wind drag on the back of your windshield. You can see that if you are very close to the windshield, you are a bit in an air bubble.
The same principle apply to the wind drag over your head.
BMW was selling the K100rt with a tall windshield. The windshield had to be cut by the dealer to be adjusted to your size.
This is one of the reason why I installed an electric one. I can set it up to get the wind on my face or to be totally (or almost) protected.
Here is the wind drag against a flat surface
Here is the wind drag against a curved surface
You can notice that the trajectory of the wind drag is more elongated when the wind hits a curved surface.
By installing a deflector, you can modify the shape and direction of the wind drag.
If you have a deflector well adjusted, the turbulence may pass over your head but hit your passenger. It may as well cool down your neck.
The angle of a deflector is more important than its size. You may want to try adding some spacers under your deflector to modify the angle.
BMW stopped installing the gaiters and replaced them with small plastic air scoop. This extra air flow helps keeping the under tank a bit cooler.
The 1983 models had problems with excessive heat under the tank. The gas was boiling. The first modification was to ad a reflective heat shield under the tank.