A 338 of some sort will give more dramatic results and generally mean less tracking. The truth is however, any decent cartridge from the 260 Rem up will kill elk with a good bullet in the right spot. A mature bull is quite a different critter than the average cow/raghead so it's wise to stack a bit more power in your corner for those boys. I'd not feel one bit handicapped with a 270 WSM however. I was up at our ranch the other day banging away with my 338 WM on targets. A neighbor rancher was moving cows down the road and stopped to talk. He asked what I was shooting and I told him... he shook his head and laughed and wondered out loud why anyone would use such a cannon for elk rather than a vanilla-grade 270! He's an old timer in his 70's and has been outifitting on elk for fifty years and has never seen the need for more than a 270.
If you've got your heart set on the 270 WSM don't let anyone talk you out of it!
