Forgot to mention that when Kondo's added side cinch straps to food pack, he suggested adding cinch straps to top of each shoulder strap.
Heavy backs tend to pull away from back, giving that slipping feeling and throwing off your center of gravity. I don't know if your packs have this feature, but if they don't you should consider it it?
 Adding them to my heavy food pack, allowed me to bring weight forward, in-line with spine and allowing hip belt to carry more of the load.
Made the balance much better as well.