That's obviously the main feature and why you would want higher octane
fuel.
However, it has the side effect of reducing the energy content of the
fuel.

What then is the actual reduction in the "energy content" between
petrols with different octane ratings?

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.

Christian.