My hunting rifle is a 26" 65SS with 156's around 2950. I built a 22" 65 Creedmoor for NRL Hunter to shoot 156's with a TB UL7. I can get just over 2500 before I see some pressure signs with Lapua cases. I could push it probably 75 fps faster but I need no pressure since this will see extreme temperatures, rains, etc.
I'm keeping the Creed in the truck on bird hunts as a coyote rifle. I'd have no problem shooting an elk at 500+ with it. It might meet your 800 yard requirement but not as well as a magnum.