I'm just going to echo other opinions that you've gotten here. IMHO with 40-50 birds, they will produce enough body heat that it is really pretty irrelevant HOW high your ceilings are. It actually sounds like the perfect situation, really.
If it would make you feel better, you could design a temporary ceiling that could be installed at a moment's notice should you need it. But I don't think you will. More important is making sure that your building is draft free. I can't over-emphasize how important this is.
I only have 4 birds and my coop was about 10-12F INSIDE WITH HEAT all last winter while it was 10 to -25F outside. That's about 5 months here in MN. The chickens were happy and healthy and I had no problems with frostbite. I'm guessing that your coop will never get below freezing with that many bodies inside.
PS - Scratch is good for creating body heat (their bodies have to work a little harder to metabolize it, thus warming them from the inside) but be careful not to give them too much. I did this and upset the balance of their nutrition and ended up with some feather picking problems.
Good luck!