Penetration isn't the only factor you need to consider.
Think of it this way, which would kill faster? An ice pic or a knife?
When a bullet expands it creates a larger wound channel destroying more tissue around the area.
Something that seems to always be missed about kpe figures is that it does no good if it is not delivered.
The best ballance is a penetration into the center of the body cavity where it will dump all the remaining kinetic energy in hydrolic shock damaging vital organs.
Air Guns, Black Powder And Archery all kill much in the same way primarily through loss of blood and secondary organ damage.
Consider a boilerplate shot? What is the bullet going to encounter on impact. Skin then perhaps a rib, then an cavity basically with soft tissues this is where you want maximum expansion and cutting force.
I'm not trying to make it more complicated sorry