I think you are right about the 351 WSL being the best and most useful of the Winchester self loading rifle cartridges and especially for a PDW rifle. what I never understand is why Winchester choose a .351 bullet diameter when the .357 diameter for revolver bullets was arround for many years.
the real overengineered rifle is the Model 1910 because it was made to compete with the Remingtom Model 8 in 35 Rem but this was not realy possible for a blow back action.
I still looking for pics showing the Winchester self loading rifles for hunting outside of the united states. that the .351 was used for pest control seems possible to me. the Destroyer carbine in 9x23 Bergmann alias 9mm largo was used by spanish park rangers for shooting wolves and this cartridge dont developed more energy than a modern 9mm luger cartridge fired in a rifle.