Had the same question for a 300 RUM in an ultra 9 vs 338 ultra. You'll need the extra volume to shut down all that extra powder in an over bore mag.
Forgot to mention, that recommendation came from TBAC after discussing my concerns and desires for a new can. You said it's for an elr rig, so probably not an issue, but some LR hunting rifles on the market these days come with 1/2" threads on magnums (fierce comes to mind). IF that applies to your situation, recommend rethreading to at least 5/8" threads if you have the barrel diameter to do it, 3/4" is preferred