People are so ignorant when it comes to defining power. Costa most certainly has power, he just happens to be quite a telegraphed striker, so it's much harder to surprise someone which is how most one punch KO's happen.
I can tell most people here haven't had any combat experience. It's the punches which you don't see coming which are the ones which KO you. Speed X Mass = Power. So I'm going to say something which will rile a lot of Sherdoggers on here, but I've always thought this - Mark Hunt has overrated power. Guy hits hard, but no where near as hard as people make him out to be. The majority of his KO's (and even then, they didn't go out cold) came from sneaky shots which his opponents didn't see coming.
When it comes to Costa, his opponents can see his shots coming, but they still go down (albeit not out cold due to seeing the punch coming). So if Costa had more speed and sneaky counters, you can bet top dollar people would be going out cold.