Higher octane does NOT mean higher energy content. Many times lower octane fuels have more potential energy but are volatile under pressure. If a lower CR and lack of boost(trying to get good economy) takes the preignition out of the equation, the higher energy content fuel will yield more energy converted into mechanical. If a computer is running open loop without MAP or other basic parameters, you are leaving a lot of power/economy on the table.
Peak economy (in miles per gallon of fuel) will occur near 14.6/1 (1 lambda).
Peak volumetric power (in HP/hours per gallon of fuel) occurs near 13/1 (.89 lambda).
When boost or very high compression is involved, often extra fuel is often added as a detonation countermeasure
or in the case of higher alcohol content to increase static VE through adiabatic charge cooling.
No matter why you are using extra fuel, the result though it may result in higher power, results in a lower power/hour per gallon of fuel.
That is that going beyond .89 lambda has an ever decreasing advantage in terms of cost per power/hours generated.