I think the lower humidity (more oxygen in the air) has a larger influence on too low octane fuel than the 0.01° timing change due to a slightly different gap. The higher oxigen content will have a larger combustion pressure rise, and consequently will knock (ping) when using too low octane fuels. With the right fuel, timing can change by about 5 degrees without you noticing any difference at all.
I.E, full power is there at about 22BTD, until up to about 30BTD, when the engine starts running harsh. Exact timing for these criteria may change a bit due to compression ratio and/or squish design, but these are the general values for well designed engines.