The proposed .338 WSM didn't eventuate because it was too close in performance to the .338 Win Mag, and would take away sales from it. By making it 8mm instead, it wouldn't detract too much from .338 Win Mag. sales.
This is exactly what happened. I know because Winchester was talking to a LOT of gun writers about where to go next with the WSM rounds, and I was one they talked to.
It was impossible for the .300 WSM to match .300 Winchester Magnum velocities AT THE SAME PRESSURE--which is exactly what happened with the testing of the ".338 WSM". This is because the WSM case doesn't hold as much powder--and despite some of BS handed out by Winchester back then case shape does NOT affect that. It can affect how consistently powder burns, but not potential velocity. (That was confirmed by independent testing by various pressure labs, both those of bullet and powder companies.
The only reason the .300 WSM matched factory velocities of the the .300 Winchester Magnum was .300 Winchester ammo was "underloaded" back then--for what reason I don't know. But present SAAMI velocities are much faster today.
