to fuel to raise octane rather than the more expensive aromatics. Hence the LOWER heat of the higher octanes one actually sees at the pump.
The usual reason for raising octane is to prevent auto ingnition caused by the rise in cylinder pressure. Hence, higher compression engines generally requiring higher octane fuels. Flame propagation speeds do tend to slow as octane rises and timing needs to be 'moved' in order to begin ignition sooner in the cycle.