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I've never been a fan of the warm air theory but that's another story.
Speaking from my own personal experience using megasquirt, there is an add on piece of software called 'mega log viewer. You essentially tell the computer what your ideal air fuel ratio targets are all RPMs and loads are, drive the car for 30 minutes, compile the data on the laptop which generates a new VE (volumetric efficiency) table. You repeat the process a few times and the VE table becomes closer and closer to your ideal AFRs. The closer you get to the AFRs you specified, the smaller the O2 sensor corrections will have to be and therefore more accurate. Eventually with this process the law of deminishing return catches up to you and you know you VE table tuning is finished.
The point being, if I do all my VE table tuning at around around 60-70f and then I try to do another thing session at 45f, the computer will want to bring down my VE table about 1-2% in order to maintain the same AFRs. This isn't a problem when you're done tuning because the wide band O2 sensor can easily correct for this in real time. So yes I also believe the engine is more efficient at lower air temperature once the block has reached operating temp. It's a more efficient air pump. The engine has to do work in order to suck in a given volume of air. Colder air is denser so the engine does less work for a given ammount of O2. Also the efficiency of of the engine depends on the temperature difference before and after combustion. The bigger the temp difference the more efficient the combustion process is.
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