pe reivers

I have done quite a few experiments with squish shapes, not only in diesel engines.
True, the carbon pattern changes towards the center of the piston. In the squish zone, the temperature cannot rise so much because of the close vicinity of cooling metal, hence less carbon.
Interesting is the fact, that a central counterpiston inside the head has three stages, progressively changing as you srew down that tommy bar:
Normal circumferential squish at low compression.
No squish if the counterpiston is flush with the head
A toroidal chamber with central squish zone at high compression.
All this depends on the way you design the head clearance.
I found best running with the counterpiston flush. Had I chosen less deck clearance, my results might have changed, though from theory I doubt that. The squish would then probably reduce power because some of the charge gets ignited too late.