Re: 7mm-08 or 6.5 Creedmore
7mm will be flatter, but not necessarily more energy. Same with the creedmore which was made as a target round, not a hunting round. If you want a 6.5mm hunting round, you need to look at the 260 rem which is a great round and very flat shooting. I dont know if I would use it for elk, but its a great round for white tail.
You wont find as much of a factory selection in 6.5mm and 7mm loaded with heavy bone smashing bullets like you will a 308. Match bullets are not hunting bulets. Dont use something like FGMM for hunting they dont expand and most times will make a clean little hole without transferring much energy to the target. Hitting is only part of the problem, transferring energy and tissue distruction is the other part of hunting bullets.
In fact for factory loadings, you wont find anything with as much of a selection as a 308.
Heavy for 7mm is a 150 grains and a 6.5mm heavy is 140 grains.
For hunting Elk you will be taking shots at reasonable distances and the 308 zeroed at 200 yards will be good for minimal holdover out to 250-300 yards. Beyond that you will have to know the real distance and know the dope for the load you are shooting.Probably shouldnt be taking 500 yard shots at elk anyway.
The 260 loaded hot will keep the crosshairs in the kill box well past 300 yards, but I dont know if I would trust the 260 to carry enough energy to take down a big elk at 350 yards...
Look into hornady's light magnum loading which is basicly +P rifle ammo. They have some premium hunting bullets loaded very hot and the extra speed will flatten out the trajectory a little. But for elk, its hard to beat the proven record of bone smashing bullets in 30 cal. The partion, the core-lokt, trophy bonded, and A-frame bullets are all loaded by a few different companies in the 308. All proven on big game, and flat enough for the ranges you should be taking shots at elk.
Just because you think you can hit an elk at 500 yards doesnt mean you should try.