As much as I like Big Show, the WMD doesn't do it for me. Even casual fans observe that even though he might throw several punches during a fight, it's only the WMD (with no discernable difference to a regular punch, except the theatrics), that wastes the guy. I know Paul Wight could most likely knock most men out cold with a stiff punch, but not if all his other punches have little effect (kayfabe-wise, of course).
The GTS, on the other hand, would legit knock a man out cold, or even kill. As would the SCF. And isn't the whole point of the Sleeper Hold to make the guy pass out? Just to name a few... However, it is my opinion that for a believabable KO, a new finisher should be used, one we haven't seen Cena bounce back from. I think that's the way to go.