i think the theory is that the acetone makes the gasoline more volatile - it vaporizes quicker in the combustion chambers. I think there might be a more complete burn in the cylinders. Very efficient cars that get high mpg don't seem to get much of an increase in the mpg, and I think it is because the fuel is more efficiently burned....the volatility that the acetone increases, isn't a factor....just a guess.
OK, that sounds plausible, at least. Fuel that is inadequately atomized will not burn fully, decreasing power and mileage and increasing emissions. However, a modern fuel injection system that is functioning correctly should have no problem producing an optimum air/fuel mixture of fully atomized gasoline and air, so I wouldn't expect any gains, either.
Why not reformulate the gasoline itself?