The stock one looks ideal. If you're talking about one of these:
Then yes, it will decrease your MPG.
The very end of my boat tail always gets filthy; I just learn to live with it.
Now, we usually don't suggest these, but I wonder if vortex generators on the trailing edge of the roof would energize the wake enough to keep the air from "sticking" to your window. They would likely decrease your MPG too (since the energy has to come from somewhere) but perhaps a 1mpg hit or so would be worth the extra safety/visibility.