From what I know that that car, it becomes quite light over the rear end at high speed.
If you were to drive around at 90 to 150mph all the time, you'd want the wing, and screw the few MPH lost on the top end for the stability gained by the wing's downforce.
That being said, its' FUCKING UGLY.
(On any car short of true race cars, these high wings are just ugly. The same effect is often gained with a lower wing, but they stick them up high like that to get "clean air")
The idea of a wing that extends the top edge of the car tends to produce downforce, reduce drag and looks tons better, but it is not up in clean air. (Think funny car drag wings, they are taking the low approach to the extreme.)
Something more like this.
Best looking add on wing I've seen yet on the Supra, and the ONLY way I'd ever add one to my car is like this one, low and pushed back so it extends the rear hatch air detatchment point.
Add in a front splitter, flat bottem pan, and rear difuser, and you'd have good looking Supra, that is stable at high speed, and still looks good. (IMHO.
)
See how far back it extends?
Should have asked to close the hatch, but you get the idea...
I'm not into the front cover rakes, but I do like the rear wing on this car.