The fences on the roof of NASCAR cars is to reduce lift if they drift so the air is coming from the side. It "spoils" the lift. Those, coupled with roof flaps, make flying NASCARs the exception rather than the rule.
just a thought on the side but could these side spoilers be the reason why so many cars and suv's are outfitted with roofrails? most of these look heavy enought not to fly off even at high speeds but their high buildup should generate enough lift to cause instability, so perhaps these silly roofracks pervent lift from sidewinds and make the car more stable at highway speeds?
I'd say the deck lid extension helps aero. It improves the effective angle from the trailing edge of the roof to the trailing edge of the deck. Also as lunarhighway pointed out the side plates continue the boattailing of the sides.