Don't worry about the velocity. There is no substitute for shooting it at extended ranges. If you will be hunting at significantly different elevation, have him shoot it at distance when you get there as well.
PS I'm a big fan of the 8mm mag. I've taken black bears, whitetails and a big Utah 6x6 bull elk at 450 yards. I shot out to 600 in Utah prior to the hunt and had to take off a couple of clicks across the board from my Missouri drops.
Enjoy your moose hunt!
