a wing will create downforce, but not hardly enough to make much difference at normal speeds. the engineers at bmw designed the back of a 3-series at a sharp angle to create a spoiler effect and create downforce. only a very well designed wing will get anymore sufficient downforce than whats already built into the car. the m3 wing and a small boot lip, in my opinion, are the only wings you will find that do this, and at the same time look good for an everyday car. a track car deserves a better, more efficient inverted-airfiol spoiler, but for an everyday car, you will look highly out of place.
a "ricer" (as some call it) wing like this is designed mainly form, not function
I suggest you look more at the m3 wings or maybe a lip on the trunk.
this is my opion, I don't want to bash you or your opinions.