I believe Weatherby is generally referring to their own ammo, which I have found is pretty close to their claimed velocities.
I own a Weatherby Vanguard stainless chambered in 300 WBY. Overall I think it is a good value for a solid rifle. It has a strong action, and though it doesn't have the best factory trigger, it is easily adjustable down to 3-1/4 pounds. I have enjoyed it, but I agree that a better stock would be my first choice in upgrades.
I might also recommend a muzzle brake
. I don't have one yet, and I got whacked in the forehead and nose by my scope while shooting prone at the range the other day. First time for me, but taught me to keep the pad firmly on my shoulder.
Good luck. Let us all know what you decide.