I think practicality takes over before theory applies. In the case of my Tiki, the decking isn't strong enough to take the weight of a large motor. Also, fuel consumption goes up very quickly. At 4.5 knots, the Tiki will get about 30 mpg (the noise is also more tolerable) at 6 knots the fuel efficiency is less and the racket will make you crazy.

I don't see that testing sailboat hulls under power demonstrates anything useful.