I talked to a couple of dealers a while ago about this mate (and very politely on that occasion) - they said that they don't really know why the Honda accessories web site says don't fit the Honda tall screen and wind deflectors together - but they assumed that it would be because there is a risk that the "turbulence" caused by the touring screen has potential to cause a strong force when hitting the deflected air from the side deflectors whereby said deflectors are ripped off.
Sounds plausible - even if they stop on - maybe there would be conflicting air streams (remember that Honda spent ages and ages in the windtunnell with the Crossrunner.
Anything worse than that - like "the bike will take off , flip and do a belly flop" is stretching the imagination. Given what they said - I decide I wouldn't invest in the deflectors - but somebody on this forum has them fitted together with a tall screen I seem to remember. Have tried a quick topic serach but didn't turn anything up.
My guess is that they do take an amount of wind off the body which is not a bad thing - but just how much difference they make - can't say.