I am from MT, and have had the chance to kill and be involved in some type of scenario that resulted in a dead elk more than most folks. Lots of different calibers, with varying results. I have seen them run off after being shot with .300 winnies, and I have seen them drop stone dead with a .243. I have killed quite a few cows with my 6's and 6.5s, not a bull (I mostly archery hunt for bulls). I keep my ranges under about 550 for the 6s and 6.5s for elk, and always aim behind the shoulder and go for a double lung impact. I actually feel more confident in my 6s and 6.5s at those ranges because they are the calibers I shoot most often and can easily spot my hits (or misses) when shooting suppressed. Important thing to remember when shooting the smaller calibers at larger games is to keep your ranges reasonable and that you are not packing the kinetic energy the larger calibers do. And be sure to get into a scenario that will give you a double lung impact.
I have said it before, but my version of what I consider to be my effective range has actually decreased since I started shooting competitively. I have rarely "needed" to take a shot over 500 yards, and when I can I just try to get as close as possible without spooking them. I have seen far too many people packing around some gigantic magnum they shoot once a year at a paper plate and assume they are good to go since it's an ultra mag, win mag, or whatever overbore is the latest and greatest. Compared to other animals, an elks vitals at even 400-500 yards are probably about 3-4 MOA. All things being equal, I would take the guy with some competency in a smaller caliber than someone who rarely shoots their "elk gun". I think a 6.5 variant you can shoot well is plenty of gun for an elk, just practice enough to be able to put two 6.5ish holes through both lungs...