"f I have more air at any given throttle position the ECM is going to add fuel to hit my AFR target. Right? Or am I missing something?"
I think you're right to a certain extent, i think the school of thought is, more air = more power, which means you need less throttle to get the same cruising speed which lowers MPG.
My cousin, who builds old carb chev engines once told me - Gasoline doesn't make the power, the AIR does.
Having said that, you are correct, wouldn't more air mean the ECM would need to adjust to add more fuel? So we are back at square 1 with power / economy