Aerodynamically speaking, that little bracket is a terrible idea.
The LEAF has some of the more complete underbody panel to keep
the underbody airflow moving quickly under the car. The diffuser
at the back plays two important roles. Properly done, and it appears
to have the max effect 10-12 deg angle above the horizontal, it
lifts/directs upward to fill behind the car. It also helps to stabilize
the airflow around the rear wheels.
Airflow around the wheels is anything but smooth and efficient.
All the wheels drag vortexes behind them. The rear wheel vortexes
move all over the place, at times curling under the car almost to the
other wheel. That little bracket is setting up a whole new vortex field
in the worse possible place and causing general mayhem in the airflow.
As for lost MPGs, I'd guess in the 1-3 MPG range, possibly up to 5.
As to placement of the stickers, it would be far better to mount them
on a flat plate lets say fastened to the license plate holder, or
sticky-taped to the back of the license plate itself.