Actually, "turbulence" doesn't lift an aircraft. The airflow over an airplane wing is generally laminar. Turbulent air comes from delamination of the airflow over a surface. Of course, I'm using turbulence in a broad, general way.

Now, some spoilers change the point of delamination from the car's rear end. This is more of a drag-reducing than downforce-producing exercise.

The Civic VX had undertrays, but it's for smoothing the airflow underneath the car. Whether this also produces downforce is a big question. Without wind tunnel or CAD modeling, we can't really know, unless those involved in designing the Civic VX come forward and put this to rest.

But in the end, the results are very hard to quantify. Our cars don't really go fast enough in a corner for aerodynamic aids to be of significant benefit. Of course, race cars (such as Grand Am Cup) are lighter than street cars, and every little 1/100th of a second helps.