std heat range plugs???? your kidding?

Thats nuts
backpressure I refer to is not exhaust related Paul... but manifold/turbo hotside related... ie backpressure acting against the back of the exhaust valves (against the springs) - there was a reason you needed a stiffer actuator. think about it.
820'c is post turbine paul, so dont keep thinking thats ok relative to the 920'egt fuel dump zone on stock hardware.. which has egt sensor in the turbo's throat... ie Hotter measurement there than where yours is mounted.
Are there any marks in the piston crows and squish deck area on the head? Looked like there was some marks in the other combustion chamber on the pictures, but I cant be 100% from your images.
bad luck, but please bare these suggestion in mind, else it will happen again!