I don't think that changing the "buttress" B-pillars will do much if any good for aero. Most of the air flowing over the car will still have that large step down to the rear deck. So to improve it you would want to have a complete cover over the "valley" there. In which case, making the slope more gradual would likely help.
Still, the drag is dependent on Cd and A, and this certainly has a large A advantage over the standard Bug body. Not sure about Cd; I would guess it is a bit better.
-soD
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