Higher octane means less BTU/s per gallon (as the octane level goes up, the actual energy of the fuel goes down)
87 octane gasoline (e0, not e10 blend) has 125,000 BTU/gallon
106 octane gasoline has 120,000 BTU/gallon
e10 87 octane has 120,000 BTU/gallon
ethanol has 84,600 BTU/gallon (however...