Initial Results say.....
VW did a decent job to start with
Here's our baseline
Ignore the fact that there's no ground effect, for a moment... Just use this as a sort of barometer for gross analysis.
Now, with our open back Kamm
Results, thus far.... With the Kamm you get the profile look of a hatchback with all the associated aero downsides. It's my opinion that adding a kammback to a hatchback is effective because you're improving the large cut off zone that is the hatch itself. By adding this to a sedan, starting at the roof - you increase this aforementioned cut off zone from the bumber/trunk to the bumper, trunk and kamm cross section.
Take a look at this, separation occurs at nearly the end- if you can't tell, trust me - I can view the boundary layer in my software, but it doesn't look pretty in a picture.
But look at the overall wake size!
Now, our baseline
Separation doesn't occur until a tad less than halfway down the glass... Which I can vouch as near accurate based on my tuft experiment. Notice how much smaller the wake zone is between the two... All images in this post are directly comparable.
Max Velocities (I can't pinpoint exactly where this is happening just yet - these values could be completely erroneous).
openKamm: 117mph
Baseline: 91mph
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As always, YMMV. Back to the drawing board - while a bit of a disappointment, I'm not scared away. Next in the pipe, closedKamm and deck lid extension.