I have to say that I'm experiencing the exact same thing, though not as drastic as several tenths of an ohm off. My DNA 250 mods are dead-on when reading cold ohms. I've taken the tank and build directly to the Therion 75C, waited for it to cool, and the cold ohms reading is far too low; around 0.02 ohms off, which makes the vape negligible. I have to manually increase the cold ohms to get it where it needs to be. From what I can tell, the 75C is different from the other boards in that I think it's compensating the cold resistance by the ambient room temp. That being said however, the room temp reading on the mod is way off; 10 degF or greater.
Wintergreen, would you mind sharing the results of your case thermal analysis? Has your experience gotten any better on reading cold ohms after running the case analyzer?