Late to this party. I will make my points, based on my ~50 years of loading;
1. There is no substitute for testing. Always take whatever someone else says with a grain of salt...even if their opinion is based on test data. Lot #s of primers/powder vary. Chambers/barrels vary. Brass varies. Loading techniques vary. It all varies!!
2. Loading density is, in my opinion, more important than total grains of powder, when it comes to the mag vs non-mag primer question. I have done a fair bit of work with mid-range loads in cases as small as 22 Hornet and as large as 470 Nitro. Any load with less than around 75% loading density with stick powder or 85% with ball powder, in a case with >50g capacity, generally ends up with a magnum primer due the better consistency afforded. But primers are never just substituted. Always work up. Lower density loads can be a nightmare with a marginal primer...even in smaller cases with less than 50gr capacity. Test!
3. Low temps definitely matter! It is simple thermodynamics. All else being equal, I will entertain magnum primers more readily when loading ammo for cold (below 0F) weather. And I always try to do my load testing at temps approximating the temps I will hunt in. Old habit from before the days of temp stable powders...less of an issue today, with some powders. Not sure if primer thermodynamics have changed as much?
Always remember #1 above, even if you forget the rest. And use a chronograph. No excuses any longer. My Oehler M33 cost an entire month's pay for a Marine Corporal in 1978. Not the case today!