i personally tried using brother's gt-wing on my old junk saga (gt-wing foc from friend), i felt nothing when i bolt the gt-wing (maybe my sense is cacat, i thought)
finally when i decided to sell the junk car, i took the gt-wing off, to my surprise the car acceleration improved slightly (can feel faster) and when hit 160kmh onwards car is flying, no longer as stable with the GT-wing
as simple as 1,2,3.....GT wing slows down acceleration by abit, and increase stablity 160kmh onwards
proved it works (regardless the GT-wing design looks like shit or not, just pure simple science)
Now then, my car with "borrowed" spoiler from subaru, mounted on my nissan (i dont really care anyone laughs, i still got my big nissan emblem) I got only 2 reasons, 1st the spoiler matches the huge size of my car's arse, and 2nd i want the downforce. Proved again it works, everytime roll more than 150kmh the feel is different, taking highway bends n sometimes zig-zag during 160kmh have more grip, no more fishtail or wobbling. (well, now with the help of TEIN, more precise handling)
my 0.5cents