So my kitchen tap "wind tunnel" may not have been so accurate
I've been doing CFD on the Malibu's basic shape. It's just a prism, as opposed the the tapered shape of the real car, and my model has a perfectly flat bottom. GM's published Cd is 0.34. I got 0.327 - within a few %. It's not super accurate, and I think it can show very good trends. I think I can say that a full underbody is worth at least 13 counts.
I've been studying various approaches to lowering drag: spoilers, vortex generators, kamm backs etc. here are my results.