I don't believe the RUM is an efficient round when you look at how much more goes in and what come out.
No offence intended but I think you are completely missing the point of a cartridge like a .300 ultramag. It isn't designed to be efficeint but rather to deliver .30 cal bullets at long range with high velocity and energy. If you want something effecient then a 6.5 Swede comes to mind. But if you want to ( and are capable of ) take 400 yd shots at heavy game like moose and elk then the Ultramag is going to be a more effecient killer even if it burns more powder per foot per second. When you point a rifle at an animal you should worry about the shot killing as quickly as possible, not what it costs per round.