Many transom sterned displacement cruising boats drag their transom through the water when fully loaded for cruising and that is just slow. I'd rather have a well designed canoe stern which eases that problem.
You may have something there. That's a much more nuanced perspective than the "part the seas and rise with a following sea" argument you usually get. As for overloaded sterns, I'd guess you'd have to do some tank tests, etc, to determine if dragging the transom is slower than immersing a bunch of stern. I'd guess (you & Bob are far more expert here than me, I'm just a pretend engineer who flew jets) that in either case, you'd have to immerse enough volume to counter the excess load on the stern. How much more or less stern ends up immersed, vs how much wetted surface area vs. induced drag (if induced is the right word for a dragging transom) would be interesting to figure out....