I ran my A/C yesterday and checked the flow on the necked down drivers side vent and it still flows pretty good. Not as hard as the center ducts but still a pretty good flow of air.
I also secured that drivers side duct to the dash with Velcro so on the days the A/C is not needed and I just want to open the vent door instead, I just pull the duct off the dash, open the door and reattach the duct to the dash.