A .54 would be fine for mountain lion.
Back when I got my first chronograph I wondered a bit about this, so I did some tests. From what I saw, with a load that gave me amuzzle velocity of ~550 mps, with loads developed at ~25 degrees C, my velocity dropped about 16 mps at about -20 C. Changes in point of impact at 50 meters weren't enough for me to notice when shooting offhand, but a better shooter might. From a bench, group center dropped about 1-1.5cm at the same range. (Had to convert the numbers in my notes to metric, sorry for the approximations.)
As Hungry Horse pointed out, lube is probably a greater concern. My tests were done using neatsfoot oil as a patch lube. Using neatsfoot oil, I've hunted in temps down below -30C without patch lube-related problems.