So this is not exact, but here is some information I took away from an old thread at evolutionm.net about the diff in the evo IX and the sti v9. The part I took from was more focused on the roof vane on the sti vs the vortex gen on the evo.
What I took from the images (attached), a single aerodynamics class and the discussion was the vortex gen made the air turbulent, which should make for less drag, but because the air was turbulent as it hit the wing, it was less effective. The vortex gen is less about making down force, and more about reducing drag, something that and a big wing to me is counter productive. Also is appears the flow on the evo air flow holds the shape of the car better and actually is mostly going under the wing and the sti air flow impacts directly on its wing. some took that to mean the evo wing was more effective and was forcing the air from the roof under the wing creating more down force. but I think the high pressure air created by the roof was missing the wing all together, as evident in third picture contradicts that with the show of the sti creating more down force.
IMO a vortex gen will work best on the very end of the vehicle, this way its not disturbing the laminar air flow over the car and just creating useless drag. So I would rather have a roof vane to help guide airflow down the rear hatch while staying laminar, then at the tip of the spoiler make a vortex gen to make the air turbulent and create less of a wake, making less drag.
These images sparked a lot of discussion on a lot of sites. here is a link to a site to see all the images with out having to join.
http://www.type-ra.co.uk/vB/showthread.php?p=71470
thanks