I have to ask, although I agree a hard knock out punch will knock you out, how are you training that punch? When I train I focus on proper head movement, and strikes to setup my takedowns (its' a fight not a bjj match). So are you training to deal with trained strikers closing the distance to a clinch?
I personally never recommend the one punch one kill approach. I've been hit very hard with no effect, I've been grazed and knocked on my butt. It's not reliable. This is why the UFC calls it a punchers chance.
That what I think atemi should be used for.
I can not remember the name of the coach that the said that. He is a relatively famous wrestling MMA coach in the US, (I can not give him credit where it is due). Basically he said that you should punch like you go for a shoot.
That being said and almost in a paradoxal way, a punch has a good chance of knock out in self defence as it usually kicks in at conversation range. (I.e. punching range)
In essence that is what Geof Thompson fence + pre-emptive strike is based on. (And that is a one punch one kill strategy; the rest is seen as secondary skill to have in case of).