I think, that as long as you can keep your hens out of a draft, they will be OK. Opinions really vary on this subject. Alot of people do not use heat lamps during the winter. One thing comes to mind is that you've got some heavier breeds there, so the will probably be fine. Your coop is rather small, and chickens generate alot of body heat. Just a few things to think about if you don't use a heat lamp. You can put a coating of vasceline on their combs to keep them from getting frostbite, and the other thing is, if their roost is a 2x4 board, broad side up, then when they roost, they can sit on their feet, which helps them avoid frostbite in their toes.
As long as your chickens are sheltered from the wind, I think, in that size of a coop, they will be OK. I'd be a little bit worried about putting a heat lamp in your coop, because as you said, it's not far from the roosts.
You didn't mention whether they have a chicken run. One thing I do to my run every year, (my run is covered), is I put tarps up on the sides, and plastic up at the door, to ward off the extreme winds, and then they can still go out and stretch their wings a bit each day.
Good luck to you!