^ The thing is, my wing is not made to create downforce, its just meant to look "pretty".
With that in mind, if the air is directed more towards the spoiler it will create a hatchback type of aerodynamic flow; which as we all know (Prius) is one of the best shapes for low Cd. of drag.
Now, you are however absolutely right about what you are talking about (creating more downforce), but only if the wing is made to create downforce in the first place. Mine is completely flat, while the evo's and sti's are at an angle- to provide downforce.
So all in all, I think they do help in reducing turbulent airflow without adding extra downforce which would net in an increase in lower Cd. But hey... only my next few gas tanks will tell
I will keep up the gas log and let you all know, and I am moving to Fort Worth next week so that will be a good highway trip that I can get good data for.