Combustion temperature and octane are pretty much unrelated. Now fuel is a complicated mixture of many different components. There is some variation in combustion temps between the different chemicals that make up a particular fuel. For example alcohol has a high octane, but a relatively lower combustion temp. One chemical that you don't want in your fuel is n-heptane. It's the zero reference for an octane measurement. It will burn hotter than the alcohol. I don't know the exact numbers, but I think that an aromatic like toluene which is 114 octane rating will burn a bit hotter than the n-heptane.