I think the stories about SB actions being softer steel are urban legend. No personal experience with them, but I've never seen documentation of a failure from inferior metallurgy. Posters on other forums have checked rockwell hardness and they seem to check out.
I think the limiting factor here is the action design is from 1898 and intended to be used with rounds in the 7x57 pressure range. Sure, they've fit bigger rounds in them safely, but if I were building high-pressure magnum from the ground up, I'd pick something else.
