Oldshooter, yes you have it right in your original post cut back and work up. Actually, since Winchester (Olin) only produced ball powders I always figured their WLP primer was a bit hotter than std primers by other makers. So they are close to magnum performance anyway to light those harder to start ball powders. At one time Winchester did make a magnum pistol primer, I may still have a few. But for a long time I used the WLP for all my loads to keep it simple. Now I use what I can get.
I'd think you'd be fine with mag primers a "slower" powder like 2400, 4227 for sure - I'd have to look but I have probably used this combination. I would much prefer to avoid mag primers with the truly fast powders like Bullseye, Red Dot etc.