They make a definite difference, though they’re not going to give as much protection as a fairing or windshield (of course).
My Buell has a small flyscreen, and it does a nice job of directing the wind blast upward off my chest.
I’ve experimented with various levels of tucking in at higher speeds, and on my bike, an intermediate tuck actually creates more buffeting, as it moves my helment into the flyscreen’s redirected wind blast…I have to bend all the way over, nearly touching the bottom of the helmet to the tank for clear air.
The Gladius has a small, available as an accessory flyscreen that would probably work in a similar fashion. The ER-6n has the same, but it’s currently only available abroad, but I suspect we’ll get it here in the states soon.