here is what i do as far as airflow
i'll start with a pic to reference off of
i also have a sunroof (the kind that retracts INTO the body)
if its really bright outside, i leave the sunroof closed (it has a solid sliding cover on the inside), and open the side windows and the "wing" windows that are right behind the door windows.
yeah, the wings probably aren't to good for aero, but i don't go over 50-55 very much (if i do i close them).
if its dark/cloudy, i open just the sunroof and wings (leave door windows up). i've found that if i leave the wings closed with windows down, stuff in the back (my back seats are folded down 95% of the time) blows all over the place. if i open the wings, i can have loose paper back there, and it hardly moves.
my favorite setup is door windows closed, sunroof and wings open. there is very little wind noise, and temps stay pretty comfortable (even with leather seats).
on the highway, just sunroof open is very comfortable. not a lot of air blows in (i have to stick my hand out of the rig to feel a breeze), but its enough unless its over 90 degrees.
of course, i have no real way to to judge mpg gains/losses yet
heres a thought. ever seen those ducts on the rear quarter windows on nascar? nothing sticks out, its basically a window with a hole in it. duct that to a convenient area.