Fabrio, I really like the look of your car in the picture you edited. It looks a lot like an Insight.
As far as any flow seperation on the back window, would you consider using vortex generators to keep the flow attached better ?
http://www.autospeed.com/cms/A_3061/article.html
You could attach them just below the roofline. This way they would reattach any flow seperation, but would also not add to the frontal area of the car and actually
decrease your mileage.