There is less engine wear and lower fuel consumption at low RPM above the engine minimum full load RPM which can vary. This is the minimum RPM where the throttles can be fully opened and not cause excess bearing wear. Lugging an engine, is bad. Spinning one fast does use more fuel when running at part throttle. High RPM camshafts will lose efficiency at lower RPM. This is why variable valve timing is now popular.
The engine mfg should have a set of fuel consumption per horsepower curves. As you change camshafts etc for higher power and RPM, the minimum fuel consumption point moves to a higher RPM. This is why some of the little 600 sport bikes get such awful fuel economy.
To me the character of an overdrive low RPM bike is less tiring on a longer trip. I will trade off a little peak power for that or get a variable valve timing bike, and live with the increased complexity. YMMV