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Some cold water:
I would not expect to see the separation points move around much at all for such a modest difference in Reynold's number. Furthermore, an increase in velocity (increase in Re #) would tend to promote a turbulent boundary layer which would tend to adhere further downstream on the surface.
One overriding consideration is that Volkswagen has almost certainly tuned-up this model in the wind tunnel. These Germans know their stuff - Hucho himself may have fine-tuned the Jetta shape. VW engineers are masters in making hardedge, almost boxy shapes aerodynamically efficient through detail optimization. Accordingly, I would bet the flow separates right off the trailing edge of the trunk lid.
As for vortex generators, I made some comments on the business intro thread about how they may help in some situations where the pressure gradient is close to the critical point for separation. Knowing VW's reputation, I would not recommend vortex generators on any of their cars except for possibly a Beetle.
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