Aero "tuning" is not about piecing together individual parts. Airflow interacts with the whole car so the car as a whole must be tested, ideally in a wind tunnel, to understand the full effect of a particular aero mod.
For example, learn from my teacher's mistake. He designed a very effective wing shape with full wind tunnel testing data. Then he installed it on the car and it didn't work so well. Downforce was nonexistent and drag went up significantly. His mistake was designing the wing without the car. The wing-to-body interaction is more important than the actual wing itself. What he found was that there was turbulence at the base of the wing, which stalled the air from ever making the wing effective.
Moral of the story: design all aero parts with the actual car to be applied on.
While the VG may be good for an Evo with a boxy frontal and side profile, S and Z cars, and especially fastbacks, have a more teardrop shape. It's not clear from Mitsu's white paper what the effect would be on our cars. Mitsu specifically limits their discussion to a sedan with a trunk. See at least page 11 column 1 paragraph 3, and throughout.
I say the best way to test the theory is to build a model and make your own wind tunnel with a big fan and a smoke machine
Or if you're a computer genius you can run a simulation.
If you want your wing to truly be effective, the rear wing needs to be higher than the roofline! This is because airflow will not drop down at the rear windshield enough to make the wing work, regardless if you have VG or not. This is worsen as speed increases. To counter this, you need a large rear wing sticking up above the roofline to be in the line of airflow.
Otherwise I wouldn't waste money on aero stuff unless you really want the parts for asthetic value.