Diesel vs 4 stroke outboard???
Your situation makes me think of all the people who replaced their Atomic 4''s with diesels, and the problems they had with the diesels not having as much torque as the older gas engine. This meant that instead of 6 knots with the gas, they got four with the diesel turning the same prop, simply because it couldn''t spin as fast. Perhaps newer diesels run at higher rpms and have more torque than before, but if not, this could be an expensive drag on your trip.