1) Your problem is not the primer (which you won't have any trouble with); it is the powder
that is the limiting factor in (very) cold weather .
2) Unless you are near the top end of the load, my experience is that Mag primers do not significantly affect pressure margins at all. (Leave the load alone, especially if/when moving into cold weather)
3) Stay away from ball powders in cold weather. Once you hit the 30's (F), ball ignition/burning characteristics degrade significantly. Varget is supposed to be less temp sensitive, but as Patrick Swaze said, "Opinons vary...."