anything 6mm and up will do the job, 6.5 would be the smallest I would use, and only if I only had that one rifle. I have never seen an elk through a scope, I kill them with arrows, but I have seen a lot of rifle shot elk die. the bullet choice seems as important as the cartridge within reason.
I have been impressed with the 270, but I would personally like a 160gr 7mag and up just due to the limitations of smaller cartridges. I would not want to still hunt thick timber with a 6.5creed. the smaller cartridges work if you have a textbook angle to shoot at, and you do your part.... all shot opportunities are not created equally.
I understand the thought of wanting a light rifle that's pleasant to shoot, the smaller cartridges are more pleasant to shoot. after about 20 prone shots with my 7mag, I don't really want to shoot anymore shots from the prone.... with my wife's 6.5creed it doesn't matter, could shoot it all day.... rifles like that are fun to shoot.
i'm hopping on the 6.5creed bandwagon soon with the savage ultralite, but it will not be an elk rifle, it will be mainly a blacktail rifle with some lion and bear hunting mixed in when i'm just wanting to cover ground.
I saw a bull killed with a 6.5creed this year, and another with a 7/08, both did fine, but it seems like a 270 and up is a little more convincing to a bull elk, but doesn't seem to change the outcome in the end.
like I said, I shoot elk with arrows, most high powered rifles will outperform an arrow in killing ability