[Angel And The Wolf]

If a boat tail extension's sides fit "The Profile", coming to a point at the back,
(Figure 1), with air pressure spilling off the upper edges as it departs the boat tail, is there any significant advantage to applying "The Profile" to the upper side of the boat tail (Figure 2), or can the top boat tail follow a horizontal line from the highest point of the car, (Figure 3) without any real drag penalty?