Alrighty then:
A roof will reduce the drag co-efficient by a bit, because it promotes laminar airflow to the rear of the car. A flat plate (like the winscreen) is as bad as you can get for aerodynamic efficiency.
However, the roof would be better still if was fitted flushly, not by press studs, and was rigid so it didn't flap about in the breeze.
Doors should be used with the hood to reduce drag, as the roof with no doors is like any other open car window.
Without the roof on, whole doors are likely to be better than half doors, which are better than nothing.
Unless, you remove the windscreen, then half doors are better than full doors (especially as you'd have nothing to attach the full doors to teeth.gif).
Better still though, remove the windscreen, rollbar and spare wheel, sit as low as possible in a half-doored car, remove the headlights and affix teardrop style enclosed fairings around the front and rear wheels (like on a light aircraft), reduce ground clearance to an absolute minimum, slap on a flat aluminium plate to the bottom of the car and fit a rigid tonneau.
Result: you'd have an ugly car with a still-appalling co-efficient of drag. smile.gif