Let's just say he has aweful attachment on the top of his vehicle, and has to raise the roof 9" to get attached flow. Let's say the length of the roof is 9'. Total additional cross wind area is going to be on the order of 3.25 square feet. This is much less than the area that is added by raising the roof on a van (which is common, and those don't tip over in cross winds), and the edges running back will no doubt be rounded over for sideways attached flow thus mitigating much of the poor handling you are hinting at.
I added a 4' long SedanKamm partial boat tail to my little car and drove it that way for over 3 months. I experienced 60mph crosswinds and saw no ill effects. Jim has added a boat tail to his Insight and driven in 60mph crosswinds with no ill effects (actually improved his cross wind handling). How big is Aerohead's bed cover when viewed from the side? I contend, it is not an issue that makes it not worth pursuing.
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