For the YS1500, last night was the coolest I had cooked with it. It got down to the upper 50's as a low, and up to about 80 at the end of the cook. I didn't notice any difference with the outdoor weather.
With the YS640, I had cooked with that in everything from 10* and blizzard, to 100* and calm. The only difference was how quick it came up to temp (it took longer to reach temp when it was colder outside) and it did use a little more pellets when it was cold. However, the food was consistent, and the interior conditions (grate temp, evenness from warm spot to cooler spot, cook time, etc) was always the same.
There are many more YS640 owners here that have had more experience and time with the pit than I had, so feel free to chime in. But I never struggled with it due to weather, and I didn't always use the thermal jacket. Actually, I used the jacket in cold weather less that I did without because the comp cart version of the jacket didn't ship until May if I remember right.