Never to late for an update.
I did three multi day camps last winter, totalling about a dozen nights in my Early Winters tent. The night time temperatures ranged from -5C (23F) to -17C (1F). In all case the first night I slept with both the door and rear window closed. In all cases there were varying degrees of condensation on the inside of the tent. The second night I opened the door and window (see picture below) and in all cases there was no condensation of any kind.
So, as unscientific as this is, I think I can say that Gore Tex breathes very little, if at all, in sub zero temperatures.
At about 2.2 kg (4.9 lb) this has become my go to winter tent. I will do some more testing this winter. <img src="/forums/images/graemlins/grin.gif" alt="" />