again, not easy to make a comment without being there but..my thoughts (in my imagination...)
I'm dismissing the "slight list at rest" as a red herring. He seems to be describing the boat heeling under power. Also, we can assume he checked for shifted weight and livewell/fishboxes full of water. In my experience, nothing on my boat can shift enough to make me lean except passengers or a cooler on deck. Everything else is bound by bulkheads/compartments. The amount of water that would have to shift outside the stringers is a lot and you would think it would run back to the bilge. Is it possible that there is hidden trapped water that shifts to outside the stringer when under power?
My similar experience was caused by trim tabs not being where I thought they were and not retracting all the way. Also, if I get on plane with motors way down, I will heel over.