http://pesn.com/2005/03/17/6900069_Acetone/
This guy is on crack. His last statements are the key - warm air over cold air? Smooth flow over turbulent? I don't think so.
Cold air is denser air, and contains more mols of oxygen atoms per liter of air. More O2 means more thourough combustion. Better combustion means better mileage AND more power.
Turbulent air generally results in turbulent fuel / air mixture, which I have to believe is what he is talking about. Why did 4-valve engines have less low end tourque, and therefore items like variable valve timing was introduced? 4-valves per cylinder allows an engine to breath at higher RPM's. At low RPM's though, you get laminar flow, which produces less vaporization of the fuel molecules, which hampers power and efficiency. This is Bernoulis (I can't spell, I'm an engineer!) principle in practical application.
If Acetone is doing anything, it's contributing an oxygen atom to the fuel mixture - the double bond is more easily broken, and the oxygen atom is available in the vaporized mixture for combustion. It's the same principle at Nitrous. Now, I'm not saying Acetone is giving you a Nitrous-size performance boost, because it won't, but that's the theory in the chemistry.