Are you seriously taking the position that mechanical efficiency is a small factor in power? Wow. Just because some people with good technique do not have much power does not mean it is not a factor.
KO power? Yes. Punchers are born. I can name a lot of boxers who can't punch themselves out of a wet paper bag and all they do is perfect punching. Malignaggi, Floyd, Algieri, Mora. Can you improve your power? Sure but only to a certain extent.
Someone like Bisping could have trained with the best trainers and perfected his punching technique and would still not have much KO power.