Well Since the analysists and testers are coming out, why not add an engineer?
There would be an effect caused by the spoiler. To say there is no effect, is well, a lack of understanding of fluid and aerodynamics.
Having said that, the effect is also not drastic either. My experiences in wind tunnel tests have showed that the actuall "inverted wing design" element is quite minimal. The position of the spoiler on the car is too low to see enough wind velocity to create large down forces. (Yes that's right, ricers do have the right idea by putting wings 6 feet in the air). Only at high speeds would you notice any significatnt down force from the inverted wing.
What people don't realize is that a spoiler that is close to the rear of the car actually changes the pressure field behind the car. This change in pressure, will help stabilize the car more than a wing at normal driving speeds (believe it or not). This is much more realivant to the driving speeds that most people see in day to day driving in a mustang (highway included).
Of course, the overall design of the car has alot to do with the design of a spoiler. The pressure field around a car is created by the body of the car, a fact that 99.9999999% of ricers don't even consider.
Having gone through all the dynamics of spoilers, for the most part a spoiler is a cosmetic enhancement, as most cars don't see the high speeds required and car bodies today can be designed to minimize pressure fields.