I seriously doubt that acetone changes the combustion properties of gasoline, but it does have a high octane rating, and it will do a nice job of cleaning carbon deposits out of an engine, which would explain the improved mileage.
Since it is oxygenated, the improved emissions makes sense.
My concern would be not only with the damage to elastomers, but also that acetone mixes well with water, and could set up some serious engine corrosion if used long-term.

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