I have the ultra 9 and I love it on short-action calibers. For magnums, I think you'd be better off with the 338, but I've never had my ultra on a magnum.
A lot of your comfort is going to depend on your environment. For example, my 16" 308win is a little uncomfortable inside confined spaces/under a vehicle even with the 9 on the end of it. In an open field it's much more pleasant. Longer barrels are even better.
Edit to add: There's TBAC's shooter's ear test video where they show the Ultra 9 to be fine on the 300wm.
I'm sure it'll be fine. There's always a bigger/better option, but at the end of the day you still have a great can that should get the job done.