I have flown at -40 degrees (yes,-40). It was done just to see if we could do it and it wasn't that pleasant. We had to put heating packs on the electronics and had the Tx in a woolen glove cover. The engines ran well though and the planes flew great in that cold, dense air.
The best part was the warm rum at the clubhouse after flying was finished.

.....I really gotta move somewhere warm!