The greater jump to the lands with freeboreactually results in a lower chamber pressure.Therefore if you increase the powder charge until the chamber pressure is the same,you can acheive a bit more velocity.Weatherby cartridges are listed by SAAMI and as you can see at the links provided,the weatherbychamber pressures are no higher than for the 270win or for the 8mmremmag,or several other cartridges.

http://www.handloads.com/misc/saami.htm

http://www.frfrogspad.com/miscellk.htm