I believe the reason why S&W guns were proofed by Winchester with .455 Colt ammo is that it was in normal commercial production at Winchester, as Colt made the New Service in that caliber, and also sold many of those guns to the British. The RCMP used the New Service in both .45 Colt and in .455, and much of that Winchester ammo was probably being sold north of our border, even before the war.
The .455 Colt gives higher velocity than the MK II, about 700 FPS to a nominal 620 FPS from a six-inch barrel. It also uses the MK I case, which is longer, but any MK I ctg. can be fired in the .455 gun. The .455 Colt is just Colt's marketing of the .455 MK I Webley or Eley cartridge.
Officially, the reason for going to the shorter case is that it handled smokeless powder more efficiently. But the cylinders would accept either case length. I think the wadcutter (Man Stopper) rounds were only loaded in MK II cases, though. They were widely used in places like India, but I think for Euro service, only the regular round-nosed lead bullets were employed. They were intended to tumble in a body, causing added wounding power. MK VII .303 ammo is also meant to be marginally stable in flight, to also create more massive wounds in tumbling within a human target. This was meant to give added killing power on wild fanatic tribesmen.
In the case of the ,303, I suspect that it's deadly on smaller antelope with a heart/lung shot, but might not hold a true course in shooting larger game, for which regular softnosed sporting ammo should be used, even where the GI ammo might be legal.
The .455 had a good rep for stopping men at close range. I think the bullet weight and diameter, as well as the tendency to tumble helped a lot. But I've read that enough British officers bought guns in .45 Colt that the supply system in India stocked that ammo, too. Also, .44-40. Don't know when that stopped. Probably in the 1920's, when new regs specified that they carry only the issued revolver. That was probably part of the gun control scheme that reared its head at that tme and just got worse.