Recently purchased the Ultra Tour (http://www.ceebaileys.com/kawasaki/concours14ws.html). I'm 6ft tall with an inseam of 34". I only have to raise the screen a few inches for the wind to go smoothly over the helmet. The annoying wind flow down the back of the neck has also disappeared. At it's lowest position the air hits the neck whereas the stock screen would hit the lower chest region.
The only grip I have is that the down force of the front wheel is reduced. Not good in wet and windy conditions.
I can't see why a larger screen would reduce down force on the front wheel. The screen is angled back and can only act like a crude admittedly steep wing increasing the downforce if anything (think Formula car). Perhaps some change in airflow is causing you to pull on the bars which might appear to give that effect.
In the down position its fine, but when it lifted a few inches higher it moves forward and out, reducing the angle at which the wind hits it.
I commute daily on the Pennines and it may, as you say, be psychological or due to riding style, but with the stock screen I definitely feel there is more down force on the front wheel