Should hold up same in cold weather as in warmer weather. They were on m5 prototypes for extreme winter testing. Salt won't affect ceramic any more than steel and maybe even less since ceramic won't rust.
Brake job shouldn't cost any more than steel brakes since it should be the same and amount type of labor, just more expensive replacement parts.
What color m5 are you getting? Can't wait to see gold brakes on the M5. I think it looks great on the M6
