Have you ever heard of brake specific fuel consumption? It's not constant for all engines.

And there are times where factors that allow additional horsepower at wide open throttle that do not reduce part throttle fuel economy.

What about examples such as electric fans and pumps, crank scrapers, combustion chamber designs that promote swirl or allow higher compression without detonation, gas ported pistons, thermal coatings, headers, etc.?

Sure toss on a single plane intake, larger cam, heads with large volume runners and part throttle economy will degrade at the expense of full throttle horsepower. Maybe economy drops at full throttle, but whose caring much about economy at that point?