I agree with the others; don't treat symptoms (getting a wing, or whatever); find the cause of your problem.

When I tracked my S2000 (entirely stock except for a Gendron front sway bar and Toyo RA-1 tires), I would run out of 5th gear (about 130mph with those tires) on our front straight, and the car was fine (assuming the nut behind the wheel was careful). My Spec Miata feels just fine at 120, with nothing special to keep it there.

Something's wrong with the car or the driver if the car isn't reasonably well planted at that speed.

 

Doesn't the rear have ~80 lbs of lift at ~100mph without a decklid spoiler?

 

Sure there's "lift." A car without special aerodynamics will always have lift at speed. But with this car, the lift isn't a real problem at 130mph if the car, driver and road surface are in good shape. Getting a wing would only mask what the real problem is.