For reference, the Army base at Fort Wainright near Fairbanks uses the XGK EX at very low temperatures. They stay on training maneuvers until the temp gets down to -40F and sometimes don’t get back in until it’s below minus 60F. This would apply to the WhisperLite stove, too.
We recommend using the Arctic Fuel Pump instead of the standard fuel pump, it’s designed for use at temperatures below 32 degrees F. If you use the Standard Fuel Pump you need to keep the pump in your clothes to keep the pump cup & fuel tube adapter O-ring warm enough to flex but the rest of the stove is fine. If you use the Arctic Fuel Pump, DON’T keep it warm; it needs to be cold. If the temperature rises to 32 degrees, you’ll need to switch to the Standard Fuel Pump.
If you are using the DragonFly or Standard Fuel Pump you should be ok to about -10F (-24C). You may be able to push this limit by keeping the pump closer to your body to keep the internal O-rings from failing in the frigid temps.