I would recommend against it. My wife also made some chicken sweaters for our girls and I would not allow her to deploy them. First, it was very cold and wet. Very bad idea. Sweaters would retain the water and turn the girls into frozen nuggets. Second, you are correct. They fluff their down to trap little air pockets which warm from their body heat so they are good to go in the cold.
Might want to get a light for warmth. I hear Christmas lights are good but have not tried that. Does not get too cold here in Austin, TX.
I will say the Chicken rescue group she donated the sweaters to really appreciated it. They use them for the birds that do not have feathers for one reason or another.....
Good luck.