243s work on elk, bulls as well as cows. So do 257 Roberts, 25/06, all the 6.5's and 270s. They all work just fine with good bullets and good shot placement !
But that's not really the question is it? Even the best of us find ourselves with a less-than-perfect shot opportunity on an animal we'd really like to take. Option one is just to pass. Option 2 is to go for it and hope both your aim and the bullet are up to the task. And even the best of us don't hit exactly where we aim, every time, even when the shot picture is perfect.
THOSE are the situations where 30 calibers and above give you the little edge that may mean the difference between a harvested elk and a lost elk, or an elk that dies quickly vs. one that dies an unfair, slow death. That's why I'm in the group of folks who use 30 cals (or a 325 WSM) on elk.
Of course just as important or more is how you shoot the round. A guy would indeed be better off with a 25'06 if he's deadly with it, compared to a 30 cal that he doesn't shoot as well. But when we're comparing apples to apples, as far as shooting accurately, there's no reason to go with a 25 cal over a 30 cal for elk. Or even a 270, I might add. Assuming you can shoot it just as well, which is a big assumption, why in the world would a guy NOT want the extra knockdown power and blood trail of a bigger round?]