I saw a 5 point bull elk killed with a .243 many years ago in Washington State prior to the state setting the mandatory .30 Caliber minimum several years ago. It was a very well placed broadside first shot at about 40 yards...4 shots later the bull was still moving across the clear-cut, the kill shot was made by my brother with his 300 WM at about 400 yards. I'm not saying a .243 can't kill an elk...I wouldn't personally hunt elk with one. That being said, states that set the .30 cal min are also out of touch...I feel my 270 WIN, 270 WSM, 7mm RM and 7mm-08 are all very capable of taking elk (and several of them have).

At the end of the day, shot placement is critical...energy becomes more important with distance and poor shot placement. But like someone else said, if this is a guided out-of state hunt...is the investment in a good elk rifle worth what could turn out disastrous?
Additionally...I wouldn't go on any big game trip without a good backup rifle, so you need another rifle anyway...I learned that lesson the hard way many a year ago on an elk hunt in Eastern Oregon.