Mag primers work fine in 40SW, and I seriously doubt you have to back off a full grain of any common powder. Remington doesn't mark "magnum" on their primers, but they advise the use of the 5 1/2 "magnum primer" in 357 mag and 40SW, specifically. And the 1 1/2 for other non-magnum small pistol primer calibers.
This has as much to do with pressure as it does case volume or hard-to-ignite powders. High pressure calibers can pierce primers if the cups are too soft. 40SW and 9mm para pressure is high enough to seal harder magnum primer cups. You might get better accuracy, ESD, or velocity using one or the other, depending on your load, but there's no real reason not to use SPMs, unless you're running light pressures using cast bullets. At lower pressures, some mag primers might not seal in looser pockets, which can lead to gas cutting on the breechface.