Power lifting is much slower than throwing a punch. You can do bench presses all day and have extremely slow hand speed. In sports like boxing, it's how quickly you react, how quickly and explosively your feet, torso, arm deliver the force through your fist. And how accurately you deliver it. I think plyo's are more important for that, once a certain level of strength is acheived. And that level of strength needed is very debatable. To do plyos, your body has to relax and contract very quickly efficiently. That is probably a more important attribute to have when throwing punches and kicks. Probably more important than loading a barbell with weights and benching as quickly and explosively as you can. It's still gonna be too slow in my opinion. It's good to train both though.