The lowest temp I have smoked so far has been between 15 - 40 degrees. I do not have any scientific proof that there was an issue, but the colder it is, the harder the smoker will work to keep temp. Yes, the smoker is insulated, but just as your house thermostat jumps more when it gets colder outside, so will your smoker. The good news is that several others have been smoking at low temps or in the snow (including my ribs from 2 days ago) and I actually had the best results I have had yet. I did 2 chickens today that I just pulled out and ate and the temp here today was about 38 with A LOT of cold wind and they turned out great.
The only word of advice I would throw out is make sure you give yourself a little more time on the front of the cook so that you can make sure you can get done by your desired finish time. You can always wrap the meat or drop the smoker temp to 130 with the extra time you have. It will keep your meat warm and help the juices to redistribute into the meat.
Good luck!