I explained a few times here how to double the warmth of most mitts, gloves with a little trick that cost less than 5 box and add barely extra bulk but every time i tried to explain people just gave up so i gave up myself
I rode at -36C with my mitts (-71F with windchill) and was just fine now with the improved version i believe i can ride up to -100F with windchill but i haven't reached this temperature for obvious reasons so i don't know for sure.