The automatic tranny is definately a weak point. It has higher losses than a manual transmission, and it makes it really hard to turn off the engine when it's not needed (engine-off coasting).
As for the maths on the front splitter, the idea is to send air over the car instead of under it. A grille block does this, and so does a splitter or air dam. You should check out
this thread, which explains front-end aerodynamics.