In their particular case, I'm not surprised as their wing on the E36 is placed nice and high. Mitsubishi published a paper on the development of the vortex generators for the Evo IX. Summary, the vortex generators themselves add drag, but reduce the overall drag of the car by reducing the size of the wake behind the car. Basically the same as dimples on a golf ball. And it increased the downforce from the rear wing because, the important part, that wing is mounted relatively low and the vortex generators were able to get more airflow down under the wing. Same with the STI. I'd say if your wing is mounted at roof level, I wouldn't bother with the vortex generators if increasing downforce is your goal. There could be some overall drag reduction if you do decide to add some, but it would take some experimentation.

 

Important note in the Airtabs description: Each Airtab® creates two tight swirls of air or vortices that combine to reduce the suction and drag at the rear of vehicles traveling at speeds above approximately 35 mph or 55kmh. Airtabs™ are also effective at the rear roof lines of automobiles that have a rear window slope of greater than 30 degrees

And the Professional Awesome ones: •Install at a 15-degree angle to the direction of airflow. PA has a picture on their website with them installed angled relative to each other. So it definitely is not just a simple install and go. Some tweaking involved.

I found the paper from Mitsubishi and it states the 15 deg yaw angle relative to airflow direction requirement.

 

Here is a great video on youtube from someone that does it for a living.
VG/Trip Strip are boundary layer control devices that are there to compensate or rectify turbulent airflow. The easiest way to maximize downforce on your car is to increase the height of the wing to somewhere close to 14-15 inch so it clears the wake from the car top. There is no need for VG/trip strip. Evo/STi has it cos it got crappy flow detachment right off the trailing edge of the car top, the VG is there to energize the flow enough so the wing can get relatively clean airflow.

 

my understanding is these roof vortex generators work best on sedans with steep rear window, that's why sti and evo have them.

for me it's risky to use vortex generator for us grassroot guys, mainly because you won't know exactly where the vortex will end up or how it will interact with the rest of the car.

How about just get a bigger rear wing =)