Though I haven't hunted elk, let alone with a 257 WBY, I wouldn't discount it as a viable cartridge. Many elk are taken each year with bows, afterall. Furthermore, Roy took a cape buffalo with the 257, not recommending that, but it shows the potential of the cartridge.
What will be important is bullet construction and shot placement. Shooter discretion is key. Don't take a marginal shot, don't take a shot you can't make. The cartridge is plenty capable, but it does have limitations;as with all cartridges.