I would recommend getting what they call the "anti-condensation mat" before you think about a winter hood. It lifts the mattress above the base adding ventilation and insulation. I've found it much more comfortable when you go below freezing with a Columbus. Yes, it is expensive, I think I paid something like $150 but the one Autohome has is made in Germany and really good, acts like a boxspring, adds a nice little cushion. I don't know much about the ones that are sold for other roof tents but I have seen some used on sailboats, not nearly as good.
Can't comment on the winter hood for the Columbus. I know that the one for the Maggiolina is good but the outer fabric of this tent isn't as windproof. In high winds, you can actually feel air coming through so the winter hood stops that.