My comparisons were based on running much the same way. At planing speeds of 18-20 knots.
Very much the same 40% advantage in fuel economy - in fact somewhat more - when running at 6-7 knots.
Seems you surely made correct choice for boat to engine match-up! Congrats on your improved efficiencies.
Mid 1960's Dad and I replaced 155 HP Nordberg Knight big straight six with a Perkins straight six. Results were very pleasing on a 37' 20K lb + SD hulled sport fisher boat. Hard to have accurately compared everything as the Nordberg basically was worn out. Suffice it to say that the single Perkins had our boat cruise 12 to 13 knots with not too much fuel use (I believe we were getting nearly 3 nmpg - memory on stats vague after 50+ yrs.).
Soooo... Yes for sure... for certain budgets, in certain boats, used for certain reasons, under certain conditions, with certain power of engine chosen, to travel certain speeds, turning certain prop-size/blade-count diesel engine is the smart way to go! And for same reasons... sometimes gasoline engine is the smart way to go. I like both type engines - my choice per boat is greatly dependent on conditions present.