Consider the tiny birds that flock to feeders in brutal winter conditions. There surface area to body mass ratio is much higher than your chickens, yet no winter has ever wiped them out. You conditions are much kinder than those birds endure, they have protection from strong winds, high energy food and liquid water without having to forage through miles of snow covered land.
Also, I like to think about times I've been outside in the bitter cold, but dressed appropriately and sheltered from direct winds. I'm actually pretty comfortable in those situations. The down on your birds is much more effective than any down jacket you've ever owned, since it's muscle controlled (they can fluff out the feathers to capture layers of air in their feathers).
So, if you want to help them, give them fresh water and plenty of food, and be sure they have a place to shelter against the winds. And check often for eggs, they freeze. If I have time, sometimes I take a small bowl of their regular food and add hot water to make it soft. They go nuts for that, just keep it to what they can clean up in a reasonable time, or it will freeze too. With a small flock, you could have 2 bowls for "hot porridge" and switch them, bringing the frozen one inside to thaw.