The straight gasoline would have a heating value in the neighborhood of 117,427-Btu/gallon vs 111,836-Btu for REGULAR UNLEADED E10.
So the engine 'see's' 4.76% less available chemical energy coming in, requiring the difference be made up in more fuel mass per brake-horsepower-hour of work.
A difference in OCTANE rating might allow a high-compression engine to operate without destructive detonation, increasing it's thermal efficiency, and cancelling the loss in the difference in Btus.
It would 'all depend.'
'Specificity.'
