I've been studying this issue and other related to fuel and air induction systems, design and efficiencies for three years now. Here is a link to the above related to the production of Nox and other emission gasses very simplified:
Emissions
It is related to the high combustion temperatures of the fuel in the combustion chamber. There is a fine line to gains in efficiency in what is considered an "ultra lean burn spark ignition engine". High output of NOX is the concern all though, CO's COV's and the such are lower. The following is an extensive yet simplified information from Cummins that also talks about Selective Catalytic Reduction systems designed to break down NOX and with Ammonia to produce a water vapor out the exhaust.
http://www.cumminsfiltration.com/files/def_training.pdf Google "NOX heat related" or any variation there of as there is lots of info to verify.
We've been able to manipulate a phase change in the fuel through the control of temperatures of fuel and air mixtures along with a configuration of a particular design to optimize performance gains and utilize more of the internal energy of today's available fuels. In general, yes, we have found raising the inlet air temperatures controlled throughout the system will provide a gain in economy, as stated from others above, at lower load demands particularly. This related to your means of warm air delivery particularly. There are problems with how today's vehicles cpu's adjust the fuel delivery in order to compensate for warmer air through the intake. It can also be detrimental to an engine through detonation if the fuel system is not operating correctly to adjust. Especially in naturally aspirated "Carb" engines as you are increasing cylinder combustion temperatures. As cfg83 mentioned, some system operating control designs will work better than others in terms of mileage gains related to warm air intake designation.