My windshield doesn't change much, only how high it goes (to push air above my head). It seems to add a little bit on my calculated fuel usage in the dashboard when it's extended and I have set the cruise control to higher speeds (90km/h+), but it's not that significant. I'd wager a number between 3 - 5% increase. The windshield has a bigger surface against the wind than my body, so it makes sense (I have the bigger windshield variant installed).

This is not something I have tested rigorously, only what I recall from personal experience on a Tracer 9 GT.