Best way to fix ABS corner curling on enclosed Prusa style printer with PEI heatbed?
Adding 20mm mouse ears was sufficient to resolve the problem using the original extrusion / heatbed settings.  I did not expect mouse ears to be required on the Benchy model, but given the lack of better advice it seems this may be a poorly documented "feature" of ABS.

Results on the original worst-case test model:



Note that the brims will not help if the bed is insufficiently leveled -- the brim has to actually merge with / melt into the base part layer, so the extruder height has to be perfect at the brim to part interface.

Example of extruder too far away from bed: