I just don't see how a windscreen is likely to increase efficiency, because to provide comfort to the rider it must necessarily increase frontal area beyond the exact dimensions of the rider.
A windscreen that doesn't increase frontal area by definition would have to be smaller than the rider. What's the point of a windscreen that doesn't prevent wind from hitting the rider?
I doubt I'd notice a difference in a tuck on my bike with the windscreen there or not. Maybe my chin would be spared the feeling of a strong man shoving my head backwards.
Only advice I have is to try the Julian method of throttle blocking to test the various top speeds at a given throttle opening on a given stretch of road. My guess is the bigger the windscreen, the slower you go, which is why sportbikes have windscreens that look like this.
If you rest your chin on the tank, your eyes will finally peer out through the windscreen, and the top of your head will still stick above it.
The problem isn't the bike, it's the rider.