Lots of elk get killed every year with .243wins and Durty Thurty's - it's definitely not a problem of kinetic energy. Most of the elk I've killed have been with "lesser cartridges" meaning less kinetic energy than the 7mm RM.
What does mixed results really mean? Elk running off? Not dropping DRT? Not recovered?
What does "marginal" really mean? Are you talking about shooting a 1.5MOA load at elk so your best bet is that you're within 6", or are you talking about taking less-than-ideal shot angles, like quartering towards or head-on shots?
What do your recovered bullets look like? The Cor-lokt is a good bullet, shouldn't have any problem taking down any elk at 400yrds or less.