What does your battery bank actually deliver in deep winter?

Spec sheets say one thing; a January cold snap says another. If you run LiFePO4 (or lead) off-grid, what usable capacity do you actually see in deep winter, and what heating or enclosure setup are you using? Real numbers welcome, even rough ones. I keep the normalized spec sheets at /specs/batteries for comparison, and I want to start recording how the real world differs from the datasheets.