Well, hot air is less dense, that's for sure. Less dense air means more throttle and rpms to create the same output; ie more load. Additionally, less dense air reduces your combustion chamber pressures similar to lower compression ratio. This is the reason you can run lower octane at higher altitudes. Neither of these lend themselves to efficiency. Further, all things being equal, cold air, which is denser, will lean a/f, as in a carburetor situation.
The reason I asked for the source is you'd think it'd be fairly easy for an OEM to integrate a load or tps based flap that would alternate between hot and cold intakes when needed if there was in fact merit to the claim. Excuse my cynicism that is borne from a performance background/culture abundant with wild claims. My Del Sol is slow, so may as well go for mpg
, but I really would like to understand this further.