Hey Bluedog,
I'm not sure I am commenting on what you were asking, but I think of shoulder turn as a load factor because it increases the distance from the barrel to the hitting area.
If you go to MLB or some other media, when a player completes his load you can see the name or at least part of it when he is at the "apex" of his load. This increased distance is suppose to increase bat speed.
There is a payoff. The barrel and knob are farther from the contact area with a front shoulder turn (load).
Bonds dad told him he was turning too much to the shortstop at load. He worked on keeping his shoulders on a line to second (more). He lowers his front shoulder at load, as does Sosa, etc.
I like this approach better than teaching a front shoulder turn. This keeps a player shorter to the zone and they lose some batspeed, but the trade-off isn't there (unless the kid is really small).