if you want to try to find real data, then you might want to see if any motorcycle mags have top speed data with and without windshields. that would give you tha answer you want on aero drag, and from there you could work your way into a FE answer.
with a windsheild I would think that the traditional ones would be bad because they push the air out, and at that point they end, for racing the wind screen pulls back almost more then it goes up, because if you don't have a way to smooth the air flow out, it will act like you have more frontal area, look at the photo of the vetter full faring motorcycle, notice the bit behind the driver? I suspect that is going to do more good then the whole frontal faring.
In the case of a sportbike, moving to a "double bubble" style screen is an improvement.
The windscreens were made, ostensibly, to smooth flow over a racer crouched down, chin-on-tank. Street rider fan-boys like myself buy them to make standard, upright riding a little more bearable, because it reduces upper body stress.
The double bubble doesn't increase frontal area considering the rider, and it sends air up and over most of the riders body-- I think the gains would be similar in effect to that of an airdam keeping air off of the car's chunky underbody.
There was an article in sport rider a few years ago supporting them, but the focus was top speed, and I'm sure the rider was in full crouch.
Here is a picture of a windshield I made myself for a trip to Alaska on my Vespa in 1978. I wasn't checking mileage back then but I assume it had a positive effect since it increased my top speed. The rear storage box I built was also aerodynamically shaped.
Will a windshield effect mileage? In my opinion, yes, one way or another. If you have a large flat shield, you will pay at the pump, opposed to a naked bike. For instance a friend of mine, that I tour with, has an extra large tall windshield on his Gold Wing, he gets a lot less MPG than I do, and much less than his bike with a normal shield. The issue is the low air pressure area behind the windshield and just trying to push that big of a thing through the air. Motorcycles in general are not aerodynamic anyway. My bike has a vent at the bottom of the windshield to break up the low pressure area behind the shield and it really helps it move through the air and get better mileage. A windshield shaped to enhance aerodynamics can improve mpg, think Bonneville, but the majority of windshields are for rider protection and not to help the bike get through the air. If I remember right CanAm took a 70 mph dirt bike and went 137mph at Bonneville with a very aerodynamic windshield/fairing set up. Joe
"Robert Barker on a Can-Am motorcycle set the 125cc land speed record at. Bonneville at 136.537 mph – a record that still holds to this day"