Like some of the guys have said, I think the term "Magnum" can be confusing. For example, anybody who thinks a 270 Winchester, which has been around for over 80 years now, isn't a magnum should think again.
I tend to break "magnums" into two categories. Trajectory magnums and power magnums. As far as trajectory magnums go, it takes a hell of a fast cartridge to shoot meaningfully flatter than a 270 Win. Something on the order of a 270 or 300 Wby starts to get there. The new short fats that are so popular this year? Forget it. Fun to talk about but no deer will ever know the difference.
And the power magnums? What hits meaningfully harder than a 30-06 or 35 Whelen? None of the magnums up thorough the 30's do, I got rid of a 300Wby as it didn't convince the animals to fall over any faster than a 270 or 284 Winchester did. I think it takes at least a 338 Win to start to get there. And personally I own a 358 Norma. In the words of a gunsmith friend of mine who built the Norma, if you're going to take a step up from 30 cal to make a bigger smack, take a step up. A 338 is half a step. The Norma is still relatively easy to shoot, and with a 250 grainer at 2800 shoots reasonably flat. Nice compromise.