I've never studied the thermocouple sections of the source code. However, I doubt it: that chip handles all the thermocouple reading and cold junction compensation and provides a reading of temperature in degrees C readable over SPI.

Where are you putting your fluke probe? Can you get it under the same washer as the CTC thermocouple at the same time?

What temperature do you get on the fluke when you set a target temperature of 190 or 200? It might be that the heater can't provide enough heat to maintain 280 degrees. That's pretty hot.

I've never tested my extruders to be honest; I only experienced problems with the HBP. I use excelvan filament, like you, and I find it works well at a set temp of 230 for PLA and 230-240 for ABS (depends on the colour). It doesn't really matter what the set temp is, if the plastic performs ok.

Have you tested both extruders? Do they perform similarly?