Treb, thanks. Good Info. I will not deny that I'm a total aero newb.
But intuitively your post makes total sense. We either want to keep the flow attached as best we can, or break it cleanly. The rear angle defines this situation dominantly.
You seem to imply that the Lancer, with its steeper angle, relies more on the latter. Being to steep for flow attachment, they were going for a clean break, so to speak. However, they apply VG's to trip flow down the rear glass! So my logic is this: If you have any flow detachment, and you can positively offset this with a simple trick like VG's, why not? I would think your ability to run simulations to examine this could be a perfect test of this idea.
The Jetta angle is less, but from your own simulations we still see that the speed of air decreases as you move down the rear glass. Perhaps the Lancer angle being more severe made a trick like this necessary where it could have little benefit on a Jetta.
While its getting hard to keep track off all the pretty pictures thus far, a simple goal, in my mind, could be this: Minimize the total amount of blue behind the vehicle, and replace the green on the trunk lid with yellow.
I don't know the scales on the Mitsu pics as compared to yours, but they seem to have done just that with the application of the VG's.
Now, regarding the deck lid extensions for the Jetta. It seems clear that the flat lid is not helping. Only extending the wake, and keeping it larger at that. However, the angled extension (same as trunk angle) looks to have a similar wake to the stock vehicle. I have my fingers crossed that VG's (or flow trip, whatever the jargon is actually supposed to be called) when placed in the right spot can bring down some yellow and perhaps replace all the green on the trunk lid and down the extension. This would lessen the wake size by far! (I hope).
I don't mean to trivialize this investigation into a simple color analysis, and I second the anticipation by tas for the force results!
Whats up with the fusion model? Can you normalize that one to those done for the Jetta (i.e. same background flow color)? It seems to have perfect flow over the entire vehicle. Regarding the Ford aero points slide, perhaps they just really simplified things. They must have also done an undertray, and that is also not listed as any benefit. Nor are the moon discs on the wheels. We know these two things would help... but they're not on the list either. My hope is that anything unlisted was just too small an aero gain to make the top 5 list on that slide. (wait, the things listed add up to the total aero gain, doh). hmmm...