Hi Mak,
There are many much easier ways of increasing the mixing of the fuel/air mixture, rather than decreasing the volumetric efficiency on one hand by heating the air and (potentially) increasing it by ensuring a better mixing.
The first would be to just fit fuel injection and the high pressure atomises the fuel very effectively.
Increase the turbulence of the flow to ensure better mixing, though this would also have the negative effect of reducing the overall airflow velocity.
There are a 1001 different carb designs and fuel injection systems to help cope with this, heating the air seems like a bit of a waste of time to be honest as the benefits would be pretty minimal compared to just putting a decent induction sytem in the engine in the first place!!