My experience is the opposite ... I think the better aerodynamics will give you better fuel economy. I went on a long ...6,500 Km ride with that windscreen. I rode with up to 4 other F800GS's at times with standard screens and at the same speeds I always
got the best fuel economy.
The other aerodynamic advantage was stability when passing roadtrains ... combined approach speed 220 kilometres an hour, cross wind up to 100km an hour. With the screen fully down it was pretty scary, with it fully up it was a bit worrying, but much better ... I was really surprised at the difference.
Of course I had the least noise on the highway, the best wind protection, and the best protection from cold and rain as well.
The downsides ... don't think there were any ... didn't get in the way offroad because I just put the screen down, didn't bother me when it was hot because I just put the screen down.
What else can I say ...it's one of the best farkles I've ever put on a bike ... if you ride on road to any extent, or are doing big trips involving some highways you should get one!
Just out of interest I have no connection with Brittania Composites at all ... I'm just a satisfied customer, in fact I have one on my XChallenge as well, and I'm really happy with that too!