On most cars, you'll typically get +0 fixed front downforce, with +5 ~ +20 adjustable rear downforce. It's practically worthless on a large number of cars, namely FFs. A small part of me dies whenever I see an FF online with a GT Auto wing. Even the relatively twitchy VW New Beetle can be reigned in without a wing.
Sometimes, the wing is great for aesthetics, even though the car doesn't need any extra rear downforce -- like the R390 GT1 road car I wanted to have resemble the GT4 Black Beauty R390.
Ninjaedit: GT4 refresher... the BTR looks awesome with a GT Auto wing. Pure. Unadulterated. Awesome.