Just my $.02 here. (Because the base of the windshield is actually a "High Pressure" area) wouldn't it be more likely to let cool outside air "in" instead of out at speed?
At say speeds of 25 mph or less, (where the slipstream of air isn't moving over the surface of the car's exterior very fast) then the heated under hood air would probably escape. But wouldn't that hot air then eventually wind up being sucked down the vents and into the interior?