While e0 is more energy dense than ethanol, I think the difference between e0 and e10 should be about 3%
In a study looking at mileage with different ethanol blends the general trend follows the energy density. That being said, your mileage will vary, The Ford Fusion tested did almost 15% better on e0 vs e10, while a flex fuel impala did just over 15% better on e20 vs e0.
http://www.speedperf6rmanc3.com/cont...evel_Study.pdf