the 300 ultra has more case capacity than the 300 WBY but i've yet to see one outshoot my WBY on the range.maybe case design has something to do with it.
Are we talking accuracy or velocity?If it is accuracy,the determining factors are rifle quality(bedding,barrel,trueness of action etc) good reloading components and proper load development.If you are talking velocity the 300ultramag will produce more velocity when both are loaded with proper powders to design pressure and fired in equal length barrels.The dual radius weatherby case may look nice to some people but it does not add velocity or accuracy.