Al though I am a fan of mechanicals I would not use one on an Elk. I've only taken one, a 5X5, but the experience told me that they are at times tough, big boned, animals that are truly hard to put down. I whacked it with a 180 gr bonded bear claw from a 300 win mag from 50 yds away and it swapped ends and needed a second shot to put down. Fixed bladed broadheads are much tougher than mechanicals, use them, elk deserve the repect that a fixed broadhead will give them.
Good luck.