The 300 Wby will make it much harder to place your shots well.
Not necessarily as much depends on the individual loads used and the fit/style of the rifles in question.
But, continuing with your line of thought, a .338WM should make accuracy still harder to achieve. The groups shoot with my .338 are not noticeably larger than those I shoot with my 7mm RM and the longest shot and kill I've ever made, an elk at 487 lasered yards, was with the .338.
