That would depend very much on the individual fuel. Obviously, higher
octane
fuels from reputable manufacturers will have been formulated to restore
the
loss of energy content, so I would be surprised if there is any on
general
sale (LPG excepted) that has a lower energy content in practice.

So increasing the octane rating does not reduce the energy content?

No, increasing the octane rating does reduce the energy content. This then
can be compensated for by reformulating the fuel to increase the energy
content to mask the effect.

Christian.