Ideally, the air should go over or around the car. If it goes underneath then it gets all caught up in exhaust pipes and suspension gubbins (unless you already have a belly pan, however over and around are still better). Upper grille block looks fine although you may wish to plug or tape any gaps between it and the bodywork. A lower grille block would be a boon also. Seeing as you live in Michigan you could probably gett away with a full one and not worry about over-heating. As for gains, I'm going to dodge the question and say YMMV
