There's no way the 284 case is going to outrun the WSM. According to the LEE reloading manual the 284 case has a useful case capacity of 3.97 cc. The 7mm WSM
is listed at 5.02 cc. The 338 Win Mag at 5.07 cc. They list top velocity with the 338 WM at 2865 fps with a 225 bullet using 75.3 gn of reloader 19. They don't have loads for the 338-284 but the 338-06 is listed at 4.36 cc. Top speed with the 06 case and a 225 pill is 2549 fps.
Noslers no 6 manual gives 2595 and 2882 fps as top speeds for the 338-06 and 338 WM. The 06 case is bigger than the 284. The WSM case would be near the WM. So I don't see where you are getting your numbers from. If you want performance out of a short case, the 338 WSM would be much hotter than the 338-284.
Have you considered the 330 Dakota. Case length is longer than the WSM or 284, about with the 06 length cases. Performance is a step above 338 WM.