What are your Januarys like, especially at night? I would guess you are a little cold and could risk pipes freezing there, but maybe I am wrong. There are some articles here about how long it takes to have pipes freeze, I'd look at those in detail. We get 20's F here in January but not for more than 4-5 hours so it's a non issue with the pump's freeze protection running. But if that were a couple of days, we would be done for at that point.
Look at historical records and you should be able to figure out a worst case for freeze time. Compare that against the articles on water freezing in pipes. And make your decision from there. In theory as long as you keep the water moving in the cold times, you should be okay with the mass of water you have, but you probably don't want to risk a pump failure or a power outage splitting pipes.