This has been discussed on LTP in the past. I wouldn't focus on the leg kick itself. Focus instead on what the muscles of the hips and pelvis are doing loading and unloading.
In my opinion, the leg kick is really all about producing load (tension) in the muscles -- so for some pitchers, a higher leg kick allows for that to happen, while for others a lower leg kick allows for that to happen.
The sweet spot tends to be a knee lift around 60%-70% of a pitcher's height.
However, higher knee lift guys like Nolan Ryan were still obviously extremely explosive to the plate because they get their hips moving faster and earlier in their delivery.