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I give VW a lot of praise, because Hucho literally wrote the book on detail optimization. But this is not something particular to VW, most every car gets a thorough work-over in the wind tunnel. If there is a little feature that can be altered for better aerodynamics that won't have much of a styling impact, any of the major automakers are likely to jump on it. Furthermore, automakers spend about as much wind tunnel time reducing aerodynamic noise as they do reducing drag. So in many cases, it may be best to leave things "stock". I also suspect that it is still true that almost all aftermarket 'aero' add-ons are never wind tunnel tested and thereby as likely to detract from as improve the Cd.
So having just looked at some Jetta photos, and knowing the subtle and easy stuff is already squared away - I would recommend seeing if you can do without the side mirrors. Next, I would consider fairing-in the back wheel wells. Not having a Jetta around to look over, my only other suggestion would be a boat tail to try to reduce the area that the wake projects. Note that these are changes that automakers normally shy-away from because they worry about customer acceptance. I hesitate to recommend a belly pan because I am not sure how it would effect the heat transfer from the exhaust pipes (I would defer to those ecomodders who have tried it). If I had a Jetta to look at, I could evaluate the cooling air flow path and make suggestions. One of these days, I will try to write a general guide to improving the cooling air flow.
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