Don't know if there is a definition of ignition speed, but the following is a defintion of flame speed -
"Flame speed This is the speed with which a flame, possibly turbulent, appears to move relative to a stationary observer. The flame speed can be much larger than the burning velocity due to expansion of the combustion products, instability, and turbulent deformation of the flame. Flame speeds of 10-100 m/s are commonly observed for hydrocarbon-air mixtures and it is possible under exceptional circumstances to have speeds up to 1000 m/s."
Additionally, I read that turbulence is the primary means of increasing flame speed.
Some previous postings noted that fuels of higher octane quality have lower heating values. That may depend upon whether you are talking about a gravimetric heating value or a volumetic heating value as there is a difference. Gravimetric BTUs will decrease with increasing specific gravity whereas Volumetric BTUs will increase with increasing specific gravity. Since in the past, premium gasoline always had higher specific gracities than regular gasolines, they would have higher BTUs (on a volumetric basis) than those gasolines with lower octane numbers. This relationship does not apply when you have gasolines with oxygenates as they definitely cause a lowering of the heating value.