James. You said "i come from a background where performers do a lot of sweating, grunting and moving around". From your posts we understand that you are also a musician who has studied trumpet and who plays extensively. This is a good thing.
I would, however, point out that not EVERYONE shares your particular background nor do they all specialize nor share your preferences in music nor musicians. I believe the discussion began with some statements concerning Dave Monette's philosophies towards body position, relaxation, etc. to which I simply added the (admitted) generalization that ALL musicians benefit from an upright, open body position.
I apologize for the generalization; as we know, ALL generalizations are dangerous!
I wonder if Miles was allowed to slouch back in his chair, tuck his chin into his chest, tilt his head sideways, etc. while HE was studying trumpet in grade/high school? I would tend to believe that he had pretty good posture until he became GOOD ENOUGH that he could "fly his trumpet pretty much any darned way he wanted".
I hope you do not take this thread personally...it wasn't meant to be that way. But you have to admit that Miles was a pretty "rare cat" and hardly the ideal model for MOST young players (or older ones!) today.