"Was told many years ago that magnum primers aren't needed till the temp goes below 20 degrees"
I heard this as well. I did quite a bit of testing this theory. I'm saddled with the fact most of my load development and testing is in our very moderate weather, except for heat. I carried my chronograph several times to both Montana and Wyoming prior to late season elk hunts. In ALL cases I had better accuracy and similar velocity with the use of 210M primers vs 215M's. This was mostly with 338 Win Mags, but also 300 Win Mag. In one series of tests with a 700STW not only was there slightly better accuracy with the 210M's, but I actually got a few more FPS out of them. Used Ramshot Magnum in the STW.
In fact, there truly was not that much difference in accuracy and if loads with 215's were all that was available, I'd never would have questioned the loads. I was also testing powders and I'd guess that could make a difference. I did lose considerable velocity with RL22 but with Ramshot Magnum in the STW and the Hodgdon powders in the 338's and 300's, no velocity loss. Very little loss with N160, which was better than N560 for holding velocity in the cold. The 'warmest' it was while testing was 12�. Most of the other times it was zero or below. Never a miss fire or hang fire.
