Let's review what you're trying to do. Redirect airflow around the top and/or sides to reduce the wake behind the windshield, without impacting the driver's comfort—yes? The traditional solution down through the years is to just chop the windshield. What kind of vehicle are we talking about?
To elaborate: A convergent duct ahead of the windshield [read: sun visor] w/ a large opening at the front, maybe 6" horizontally taking air off the top of the windshield with the exit a vertical slot maybe 1" tall that blows over a Coanda nozzle at the back of the windshield header. This passive duct, like BMW uses, would not be as effective as pumped air. An augmented Coanda nozzle would look like this:
The thing is, if it works the driver would need goggles.