JeffE,
While I don't have an RX I do have a Crow Magnum IV(another gas spring type). I have never experienced any velocity loss due to temperature changes but then again I never did any chronographing in cold weather. I have found a very small drop with my PCP's at colder temperatures(about 2%) but not enough for me to worry about it. I like the added velocity of the gas spring and of the .20 over the .22. My Crow Magnum gets over 100+ fps. more. For hunting, you won't go wrong using Crow Magnum pellets is whatever caliber you get. They expand, even at lower velocities, and penetrate giving quick kills on body hits. They are the only pellet I use for small game hunting(rabbits and squirrels). Lawdog