Any! They plug wells need to be blown out with compressed air, spray in some brake cleaner first then use compressed air. I doubt just dirt is the cause of you misfire though. Try identifying which side is the misfire (from the exhaust) and then pull one plug HT lead at a time to identify which cylinder.
Once you know the cylinder, look at the plug and if that and a new lead do not help, look at the injector loom and if needed the injector (if hard use and strong injector cleaner in the tank do not fix it after a tankful or two).

Have you refurbed the injectors and the injector loom yet?

 

injectors and loom are brand new, I cleaned the plug wells to the best of my ability when I did the plugs, used a nylon bore cleaning tool and q-tips to get as much crap as possible out but the mating surface itself seems pretty rough.. I know they sell a tool to recut them but it's prohibitively expensive and probably requires removal of the heads

Pulling plug wires seems to indicate that it's firing on all cylinders, all injectors can be heard clicking open/closed with a stethoscope.. what really has me stumped is how well it clears up with rpm, even just at ~1200 it purrs like a kitten

 

I think giving it a good caning will fix matters! Also try some valve stem cleaner as well, maybe?

 

Agree with Greg.
Injector cleaner, 1 bottle added to 20ltrs (approx, no need to be silly with measurements), and DRIVE it.
Take a 2nd bottle, and add that when the beast has drunk the fuel in the tank.

These are not city engines, and more so not now with all the rubbish watching our every move.

Worse down here, as we got the 12.5:1 engine.

Your in the US, so Vegas road trip, enjoy.