I'd like to attend a double blind test of this kind of thing myself, regardless weather it's interpreted as Ki or bio-mechanics. The psychology part you touch on to me is also very much of interest, how much is self deception involved, and a double blind should sort that out.
The clips mook jong man linked were interesting, I'd love to participate in something like that to experience first hand. I can only compare with some of the training I do now in Hapkido and really teaching my mind that force and strength is not everything.
So for the OP, as long as structure and intent is maintained during sparring or a RL encounter, then a version\interpretation of ki and bio-mechanics is used in "combat". - looking to the more experienced participants of this thread to agree\disagree.
On a side note, so if you were to describe low level ki\mechanics, Wing Chung, keeping structure and center line is comparable to Aikido circular motion and reservation of energy.. really not sure how to phrase what I mean here. (This is two examples of what the OP was asking?)