One thing I have done is fill the body gaps between panels with rolled rubber (black silicone, actually) gasket weather stripping. Adhered with self-stick on the inside / unseen parts of the panels, of course. Side benefit of that is reduced wind noise, and water entry between panels. Didn't really do it for improved aerodynamics, but that's also little benefit too, I'm sure. Car looks like it should have come this way, definitely more finished appearance.