Expert opinion is of a guy I certainly trust as I've seen dyno and MPH results of his readings. His reasoning on these is exactly as you mention, looking at the threads aren't showing any heat and there is still a decent amount of carbon on them, aka not hot enough to be self cleaning. Be mindful as well, the LS coils have pretty hot and long duration sparks too, so the plug is the limiting factor.
Delco 44 and Champion 12
Rebuilding an OHC Pontiac 250 with EFI and a Turbo