The WSM cases are going to be slightly smaller in case capacity as compared to the .300WinMag while still being substantially larger than 30-06 capacity. If you've had no issues with 7mmRM levels of recoil, the .300WSM shouldn't be much different in the same rifle. Websites like Midway, Natchez, and Graf's can tell you what's out there for factory ammo options and how readily available it will be. I don't see it dying off anytime soon, but the WSM rage isn't what it was 15 years ago.
FWIW, the Montana action was supposedly designed to house up to a 3.1" OAL in the magazine, if I recall correctly. Their action was seen as a good fit for the WSM as compared to a 2.83" magazine. I'd not be scared of a .300WSM if I wanted something at that power level.
