Maybe it would be useful for you considering the purchase of TTech spoiler for F800 gs. It did not worked for me and I think the issue is the hight of the rider. The product itself is very good and fits very good to the oem windshield. The result I got was less wind on chest (good) but much more wind on the top of my helmet and it was not a clear wind but turbulence / waves resulting with a strange noise inside my helmet (schubert C3) and head. The noise for me is very difficult to cope especially on longer rides (even with good earplugs). When I took off the spoiler and get back to standard oem there was more wind on my torso but it was much more smooth flow of air.
I am 180 cm high (so if think it is 5' 10''), inseam 80 cm (so 31'-32'), standard seat. When I hide a little bit behind the oem + spoiler the comfort was much more better (less noise and wind on helmet). So it maybe a cool cost-effective solution for those who have lower seat or who's hight is lower than around 180 cm. From the posts in this thread it looks like it maybe an option for people higher than 180 cm as probably the air flows differently over the helmet/neck. The 180 cm around hight seems to be a tricky one for that spoiler.
Hope my opinion would be useful and save you some $