I put a Cee Baily clear 15" vented on mine
I love it. I look just over the top, and at most speeds it keeps me in a quiet, turbulence-free zone.
I do believe the vent helps. Not so much for air to the rider, but for streaming some air up along the inside of the windshield. It helps to smooth out the airflow coming off the top, cutting down on buffeting.
The Cee Baily vent does this even when closed, since there is an open slot at the top of the vent directing air upward. I've noticed an increased benefit when opening the vent one notch. Any further open than that passes the point of diminishing returns, IMO.
Since I always wear a full face, any actual venting through the windshiels has little value.