I've been tracking my fuel economy since I got it, and I can say that my fuel economy went up as soon as I installed the large screen.
Obviously this depends on the angle and how much profile drag is generated. I have it so the wind hits me at neck height (5' 11").
I now fill up the tank with 260km on the tip metre (around 15L used) - this is in both city and hwy travelling - but I don't thrash the balls off it all the time.
Last week I clocked up 1755km in 20hrs (16 travelling time), the screen works well but still wind is a factor is you intend on travelling above 160 km/hr... I guess if you want to eliminate wind all together a big tourer is the go (eg honda, BMW, Kwaka..) since they can create a bubble around you ...
The plus side is I have attached aux lights (Hella FF50) to the bracketry used for the large screen .... see my thread on aux lights for some photos....
hope this helps....