[wyatt]

Originally Posted by PaleMelanesian

On the other hand, the effect might be like this: Aerotesting - Sunroof open vs. closed. - CleanMPG Forums

Tape some strings to your side windows, front and back, and see if / where you have attached and separated airflow.

Interesting... I will second the tuft testing idea. We could hypothesize all day about whether or not they make a difference, but the tuft testing should tell you outright.
One thing you could try is only removing it on one side of the car, tufting both sides and seeing the difference in real time.