[Stan]

Based on extensive CFD studies we've done for racing customers I think having just the outside skirt between the front and rear tires is your best (lowest drag) option. That's because the outside skirts keep air from migrating down from the sides of the car to under the car. This reduces the disruption of the airflow and added turbulence under the car, and hence reduces drag. That's why nearly all racing rules prohibit skirts down below the bottom of the car frame.

Your "tire spats" do add a fair bit of extra wetted surface. A trick we use on race cars to reduce wheel opening drag is to add a small "Gurney Flap" to the leading edge and top of wheel openings. This photo shows a roughly trimmed left-rear fender we made recently for a customer's sports car.



You can see that the bodywork turns out 90-degrees right at the edge of the wheel opening. That turn-out needs only be a centimeter or so tall, but it does two things for you. First, it creates a low pressure area outside the wheel opening which helps evacuate air from the wheel well. Second, it diverts the freestream air away from the wheel and tire, thereby reducing the drag component of those pieces.

There's no free lunch, though, aa the flaps do add incremental drag, but possibly less than the spats they would replace.

Stan