i can't imagine the stock roof crossrails/setup to take away any significant MPG.
they look pretty aerodynamic...since they are pretty thin.
if anything, Yakima or Thule with their circular or box-shaped cross bars take away from the obvious resistance to wind those would pose. I had some on my old Honda Element and when I took them off noticed a 2-3 mpg increase. I can't imagine this thing and flat cross bars taking away more than 1 or 2 MPG tops!