If you will read some of the statements in a few of the forums here, you will read about lost elk using the 6.5 Creedmoor. I will refer you to 'Ballistics Studies' which is a website out of New Zealand that a ballistician and hunter runs. He has done extensive wound analysis and impact studies on animals he and those he has guided have killed with various combinations of bullets in calibers ranging from .224 up to .475. Read his material on bullet diameter and weight.
I would not hunt elk with a 6.5 caliber anything. The bullet doesn't have enough mass to penetrate heavy bone at any distance past (maybe) 200 yards. I live in Colorado and go out every year for elk. I use a .35 Whelen and a 225 grain Sierra or a 250 grain Speer. My backup rifles are a .300 winmag or a 30-06. The lightest bullet I load in them for elk is a 180 grain Sierra Pro-hunter. At 400-500 yards, a 140 grain 6.5 Creedmoor round has a velocity (at 7,000 ft.) of about 2100fps. It has about 1,350 Ftlbs of energy. You've just shot a 600 lb animal with the equivalent of a .223 at around 50 yards. With a 300 winmag using a 180 grain Sierra GK bullet, my MV is about 2450fps at 400 yards with a starting MV of 3,000fps. The energy is apout 2450 foot-pounds. (.308 Win. at about 50 yards) The bullet mass is about 20% greater, and it makes a considerably greater entrance and exit wound. With the Whelen and a 225 grain bullet at around 2725fps starting velocity, I have about the same velocity at 400 yards but my entrance and exit wounds are much larger, and because of the bullet mass, it penetrates heavy bone and doesn't deflect.
Elk are pretty tough. Go with a round that will penetrate and exit at 400 to 500 yards and give you a good exit wound. The larger the better, because you may have to blood-trail him. Also, a large sucking chest wound will put him down much quicker than a small one. A 6.5 Creedmoor is accurate, moderately flat-shooting and easier to hit with. It can kill an elk, but so can a .22, and if you hit an elk with it properly, it will work. However, the elk may be in the next county when it dies, and you won't have much of a blood trail to follow if you have to trail it. A .300 or a 30-06 makes a nice large exit wound and leaves a good blood trail. My Whelen makes a very large blood trail, and an enormous sucking chest wound at 500 yards. Pick a round that will do the job if the shot isn't perfect at 400 yards (which it probably won't be). Go with the 30-06 or the .300 winchester magnum. (or the Whelen, which when handloaded just about equals the .338 Winmag.)