Assuming you size the VHB patches correctly, and that the panels are no more than a couple of inches from the roof, you don't have anything to worry about. The windshield actually pushes the air up and away from the roof, so at speed there isn't much of a "wedge" affect on the panels leading edge. You do not want the panels to extend past the roofs leading edge and over the windshield, as this can create much larger aerodynamic forces, and may even impact fuel economy.
Some of the air will be pushed up and over, but it won't be a dead zone by any means. I had a yakima roof rack mounted 3.5 inches from the windshield of my commuter car and it caught A LOT of wind. It created so much drag my average MPG dropped from 32 to 28 in a four-banger and the wind noise it generated was really, really really loud.
The solar panel would have to be nearly flush mounted with some sort of air dam between it and the windshield to truly deflect air up and over the unit. Anything otherwise would cause it to split the air above and below it.
Take a look at this wind tunnel test of a van.
https://youtu.be/oMsg8KorxJA The only true dead zones are just behind the vehicle, or in pickups, in the bed of the truck by the cab with the tailgate down, or in the whole of the bed with the tailgate up. Even then, they aren't true dead zones as the air still swirls, just not as fast as the air flowing over those pockets, or dead zones.
Even though I think I'm making up my mind for a gutter mount rack for optimum flexibility for myself and a future owner down the road, this thread could help someone else. For what it's worth - my intention was to consider the aluminum angle iron and VHB tape for 1-2 panels with dimensions roughly 40"x40" and to mount this at the leading edge of the roof, very close to the windshield on a lifted E350. Would forces from wind at 80mph be something to consider with that placement?
Personally, that would cause me a lot of anxiety. I would be worried about hitting a pothole at speed or large gust of wind at speed.
Also, if you are looking for a non-permanent solution this isn't much better than simply bolting it to the roof. The tape will be very difficult to remove without peeling your paint off. For non-permanent I would go with a gutter rack. Plus, with the rack you can always mount for stuff up there, or use the rack as a tie down point for an awning or a windscreen sidewall or similar.
