the fact that air is caught and redirected by that thing will add a little drag, but as that air is expelled where you'd normally have a turbulent wake it might actually reduce the size of the wake slightly,
also it doesn't really add any frontal area.
on the other hand if the plastic bit is removed the downsloping roofline looks like it will be aerodynamically favorable.
if you can take it off fairly simple that that's worth giving a try, if not, enjoy the clear rear view
