As far as I am aware, if RPM is maintained from stoich to lean burn, then the throttle/pumping losses are the sole (most significant) contributor to improved fuel efficiency. We can get into semantics about maybe thermo efficiency, combustion pressures, harmonics and tuned intakes/exhausts but many of these changes have little effect in say a 200 RPM band in the low region anyway.
'Power can be increased or maintained in other ways' again..... of course power can be increased in other ways such as with more fuel, lol. More Ignition timing in a modern engine will not improve power or fuel consumption measurably.
I'm not rehashing the last point either, I understand why lower RPM's are desired during lean burn, so save your breath.
