well I have read through this thread and thought I'd throw in a few thoughts

I have a friend in Green River that started hunting elk in his young teens with a Winchester M88 in 243. He killed 8 elk with that rifle. He said the use of a good bullet is the trick (no big surprise there) He still have the M88 and he still stand by his conviction that a 243 is fine if you are a good shot, and if you use a bullet that will penetrate well and not blow up. His comments mirror what I have seen as an elk hunter and guide for over 40 years.

However the other side of the coin is that he now uses a 308 and a 270. When asked why he simply said "The 243 was good but the 270 is better". It shoots a heavier bullet and penetrated deeper making a larger hole.
I asked him how many of his 243 bullets exited the elk he killed and he said about a third. When asked about the 270 (he shoots 150 grain Partitions) he said "all" That's been my experience too.

So I would never tell a good marksman not to use a 243. Just get good bullets for it.
But if you were to go out to buy a rifle for use on elk I would recommend a 6.5MM or larger, just because you get into bullets of 140 and larder as soon as you get to the .264" diameter and the extra weight of the larger bullets can help in breaking bone and still going deep. I have never been one that believes you need a big magnum for elk. I do use them when I like, but that because of only 2 reasons. #1 I own some powerful rifles that I like, so I hunt with them. I don't need that power, but I just like the rifles.
#2 many times I hunt where there are LOTS of grizzlies. Then I do feel like I should have some power. I am not hunting the bears, but I understand they may want to hunt me. My 9.3, 375 or 404 would make me feel a lot better shooting at something big, very close and getting closer fast. But years of experience killing elk have taught me that you don't need a huge magnum for elk. if you like them and you shoot them well go ahead and use them. But don't bad-mouth those that use a 243. You may have to eat those words if you do.

I hope to kill my elk this season with a 6.5X54 M/S. Not exactly a powerhouse compared to my 300 mags or my 9.3, or my 375. It fires a 160 grain round nose bullet at about 2250 FPS. But it shoots as well as I can hold it, and the penetration is good. So I think it will be fun to hunt with it, and the 6.5 M/S had a wonderful reputation on large thin-skinned game in Africa and India for many years, and I am sure it's as good today as it was in 1920. In energy, it's only a smidgen ahead of a 30-30, but shoots much flatter.

I can always grab a scoped more powerful rifle is I hunt the elk and feel like I am not able to use the old Mannlicher as well as I should. In early and late light I can't see as well as I did when I was young, so I may have to concede to my age and grab a scoped rifle, but I'll try the 6.5 for a few days before I do.
Should be fun.
Killing game is much more about the hunter than it is about the tool he is using.