here's a few more idea's you might considder,
the green dams would have most effect along with possibly some sort of side skirts. measuring the height of these dams on normal vehicles could give you an idea on minimum ground clearance for normal driving
making the wheel wells more tight would also cut some turbulence as would adding smooth hubcaps. rounding the A pillars could help to, although that's a bit speculative.
anyway, just a little fun i had after seeing your great work.