It will probably help, but as Frank said, I wouldn't expect it to be noticeable. If a full rear wheel fairing increases mileage ~2-3%, the benefit from such a small lip will probably blend into the background noise of FE variation. However, remember that small increases add up.
OEM's that actively seek aerodynamics generally set the tires deeper into fenders, lower the gap distance, and fair the trailing edge. The best examples I can think up of are the Honda Insight and VW 1L. If you look carefully, you'll notice how the rear of the wells are faired differently.
Honda Insight
VW 1L
Idealized Open Wheel Well
- LostCause