Rob, higher octane fuel does not burn slower than lower octane fuel. This is a misunderstanding that has become widespread. There are small variations in how fast a fuel burns, but these differences depend more on the specific formulation of the hydrocarbons in the fuel, rather than the overall octane. And these differences are very small, so they do not appreciably affect the timing advance requirements.
The idea that higher octane fuel burns more slowly comes from a misunderstanding of how the fuel chemically alters as the pressure and temperature increase to form pre-ignition compounds, which then burn when the sharp increase in temperature of the flame front arrives. The formation of the pre-ignition compounds is slower, which reduces the tendency to detonate, but once the flame front arrives, it burns at the same speed.