why are my plug electrodes doing this?

Is the residue from heat, or lead? I would guess its lead depositsand it probably isnt a good indicator of the heat that was present. I do know that in aircrft, once you start leaning mixture and pushing EGT, there isnt a lot of leeway once you near peak. IOW, near peak EGT temps, less than 100F seperates the difference between an engine lasting virtually forever, and burning off an exhaust valve within seconds. I would garner your reaching the ragged edge.