Not sure if these plugs have been in your engine when adjusting your mixtures from the start?

When did you take the plugs out after a W.O. T. run, steady state cruise, idling for a while then pulling the plugs?

If you had run rich w/your plugs while adjusting your EFI, it is kinda hard to burn off the sooty black carbon build up when your mixtures were rich @ one point or another.

I would not necessarily go to a hotter plug, those plugs are a fairly warm plug. Might have old carbon build up when the mixtures were off?

What is the A/F ratio @ idle?
Having an advanced timing like you seem to have will make the plugs run cooler. Maybe run it a bit leaner @ idle?

I run 14.7-15.2 A/F range on my Syclone while idling.

I also run 1 -2 steps colder plug than stock, but it's turbocharged & I try to stay away from any detonation.

12 port SDS EFI