Is 350lbs the weight of the incoming carcass?
585lbs for a full carcass is as much as an AK moose. Is that what the butcher scale said?
My experience is this... a mature bull will net you 220-260lbs of boneless cut and wrapped meat, commercially processed. You'll get slighly more if you drop the entier carcass off with the processor, and less if DIY.
A typcial rag horn 2.5yo will net you about 140lbs.
A 3.5-4.5yo will get you in the 160-200lb range depending on time of year.
A cow will range from 90-140ish pounds.
This has been my experience with elk anyway. I cut wild game for a frew years commercially, have packed/processed a pile of my own/friends/family.
I would be intersted to hear from some of the guides/outfitters here that kill big mature bulls every year. I would say for every 20 bulls one would be a mature bull, and these were all killed after the rut. I imagine the big prerut bulls are a little heavier.