It feels wierd but in my own study/experiences I came across the ideas that you mentioned SP. I've been thinking about isometrics a couple of months ago. One realization that I had, is that it won't affect the movement of a punch per say, however it will make a difference in terms of force at the moment of impact.

Say for instance that if your objective is to kick a door off it's hinges, your training to improve your speed/explosivity using plyometrics or other means will help with initial part of the movement and with the movement itself, but at the moment when your foot makes contact with the door, that's when a strong isometric contraction can make a difference. It's what I've been thinking, although I don't know if this line of thinking is correct.

I also saw an example of this in the book "How to be an Ass-Whipping Boxer" by Jay C. Thomas. It's worth looking exploring.