Older thread, but a couple things...
The stock wings and spoilers do basically nothing with regards to downforce. The main issue is that the shape of the roof requires an effective wing to sit much higher to work. The 2000 Cobra R is a good example of a functional wing for this style of car, as well as the more serious fox body race cars.
Secondly, the reason why these cars feel unstable at speed is because (as is common on a street car) they create lift rather than downforce. The only way to reduce this problem is to reduce the amount of air going under the car using a valence. A splitter can also be combined for further effect.
The downside to this on a street car is the reduced ride heights required to make these pieces effective means they will likely be damaged with normal street use.
Lastly, the ranger air dam serves a purpose, but not in reducing lift - it is actually about making the cooling system more effective by directing more air through the radiator, which can be an issue for Fox mustangs on road courses, especially if they are making more than stock horsepower. The positioning of the air dam is a source of lift, and the ranger dam only exacerbates the problem.
The best solution is to convert the car to a front breather by sealing the radiator off so that it takes air in through the front fascia and eliminating the stock air dam. This is a requirement when adding a valence to the leading edge of the front bumper because the reduced air under the car would cause overheating in the stock configuration.
