I read this googling "Ultra-mags": "In field test the .340 Wby clearly out performs the 300 Ultra mag, .338 Win mag. and even rivals the larger .375 H&H providing a much flatter shooting harder hitting performance."
Not sure if it is totally true or not, but it makes sense. If you are going to put up with the increased recoil of a .300 RUM. You might as well get the .340 Wby. A .30 cal bullet (my favorite caliber) is still a .30 cal no matter how fast you drive it. If we are bringing big bears into the conversation, it seems a .338, .35, or .375 bullet is superior.