^^^ OK, here goes my perspectives since the thread OP might still be prepping his own car...
As to B-M's windshield vents, I've left mine open since installed. At any speed from trail crawling to short highway stretches, can't really say I feel the effect of those vent slots. While driving, I can reach forward and do feel air flowing, however not enough to "stir" the air in seated position to offset engine heated air drawn forward from the rear outboard bulkhead openings. I really like B-M's windshield but couldn't deal without more airflow, so installed Can-Am's "Wind Deflectors" on both sides (requires drilling 2 holes per side in cage a-pillars for provided nutserts). Two days ago, we did 40 miles rough terrain trail inspection run, which was my first test of those deflectors. Worked much better for me, although I did rash the passenger side deflector on a narrow trail with intruding tree/brush. Bottom line, I prefer the significantly greater airflow using a half windshield but don't miss the dust nor wind noise picked up by my comm mic.
As to B-M's windshield vents, I've left mine open since installed. At any speed from trail crawling to short highway stretches, can't really say I feel the effect of those vent slots. While driving, I can reach forward and do feel air flowing, however not enough to "stir" the air in seated position to offset engine heated air drawn forward from the rear outboard bulkhead openings. I really like B-M's windshield but couldn't deal without more airflow, so installed Can-Am's "Wind Deflectors" on both sides (requires drilling 2 holes per side in cage a-pillars for provided nutserts). Two days ago, we did 40 miles rough terrain trail inspection run, which was my first test of those deflectors. Worked much better for me, although I did rash the passenger side deflector on a narrow trail with intruding tree/brush. Bottom line, I prefer the significantly greater airflow using a half windshield but don't miss the dust nor wind noise picked up by my comm mic.