I have been using VV N570 for a couple years now in my 338 Terminator. I have had no issues with the velocity in summer or winter and have started documenting Velocities at different temps. What I have found so far is, that when both ammo and rifle are allowed to cool to ambient temps, I actually picked up a little velocity in extreme cold conditions. This held true for a cold bore and a follow up. Rifle and ammo was left outside for over an hour for each test. I believe the slight gain in velocity was from the contraction of the barrel and believe the VV N 570 to be pretty temp stable by itself.
Jeff