Maybe if its not overdone. And it still seems like a bit of band aid fix to me. In other words, I wouldn't use one until I ran into actual stability problems. Keep in mind that while swerving into the wind is more desirable than swerving away, if the car already has no problem with stability in crosswinds, there is no need to make it move one way or the other. The idea is aerodynamic neutrality. Keep in mind that you add a boattail to the back of the car, your already moving the CP back. Now, if you made your boattail really heavy then it might move the CG back significantly too, but if you making mods that add that much weight to the car your kind of defeating you purpose in hypermiling
I think at the beginning of this part of thread, you were thinking off adding the vertical fin because you thought that the boattail might decrease stability, and looking at what I've learned on this thread, I think that's pretty unlikely. It'll probably have a neutral or positive effect on your stability.