BTW Bob, I followed your link. And while you are somewhat correct that most streetcar spoilers are just put on for looks, you also hold some misconceptions.
First of all, it matters not a whit which wheels drive the car. If the car, in stock form, suffers from rear lift at high speeds, this is a bad handling situation and needs to be corrected, no matter which wheels are driving. If the whole car suffers from lift, then a front air dam/splitter needs to be added to provide downforce at the front, as well.
How much force? Well, it depends. Downforce is a direct tradeoff with drag, so more downforce means lower top speed, but faster cornering speeds (up to a point.) The faster the track, and car, the smaller the spoiler.
On the street, when factory spoilers are not just for show, they almost always serve the purpose of cancelling lift or reducing drag. The famous biplane wing of the Merkur XR4Ti, for example, reduced drag. (The big "whale tail" of the Cosworth, however, increased drag and lowered top speed, but gave real downforce). Likewise, the Infiniti G35 features functional aero add-ons that move the Cd from .27 to .26, and reduce aerodynamic lift at the rear. And the trunk lid doesn't even buckle.
http://e30m3performance.com/myths/splitter/splitter.htm 
