Sufficient...usually/often, sometimes not
Ethical...judging by the number of negative answers...possibly not. But, it is legal and the Gamd & Fish folks think that it will work, so actually, that makes it ethical, as they make the rules. (as a side note, on a youth hunt, run by the CO-DOW, they specify, I think, .270 or bigger)
I have used the .243 on Elk, it took 3 shots. The first one was a tad high, taking off the top of a lung and hitting the spine and then disappearing. If it would have gone through the spine it would have made my life much easier. I can tell you that a deer/cheap .243 bullet will bust an elk's shoulder to smitereeens and then disappear. (from my locality I was able to watch the elk go lay down and also see that it was blowing out blood around its mouth, I shot him again and then approched, as I neared, he stood up, I blasted him again...he promptly ran into a deep crevice of a thing and died...he was a bear to get out of that hole). I found one bullet in the elk. They were all fairly decent hits.
Now, if you handload, and use a Barnes or Partiition, that changes things. I was being cheap and it really, REALLY hurt me.
I know a guy that knows of a ranch/outfitter in CO that used to, or still does use 6mms as their loaner rifles. They have the boys test their guns on the first/arrival day. If they can't shoot their .338 WhizzBangUltraMag (WBUM) then they tell the hunter, look, somthing is obviously wrong w/your scope/rifle. We don't have time to sort out the problem but we have a 6mm and if you take your time and shoot squeeze the trigger and PUT THE BULLET IN THE RIGHT SPOT it will kill an elk. The guy shoots the teensy-weensy 6mm and instantly the guy turns into a SGT York type of shooter. The guy then shoots the elk in the lungs and then falls over dead. So, they have good success w/the 6mm (which is pretty mush the same thing as a .243).
The secret w/any elk gun is the bullet.
John 14:6