Piwoslaw posted a graph from Piechna's book:
This agrees with my tuft testing as well. For minimum drag, 4° looks great. Even up to 7° looks fine. But things do not rapidly fall apart until 15°. From 15-30°, it looks like you'd be better off just not having an underside. I will avoid these angles.
Of course the actual numbers may be different because the wood block isn't shaped exactly like an Insight, but the trends should be the same.
SI2, I've left the exhaust and wheel wells wide open. Things don't look good except down the center of the car.