For its energy density to matter, it would have to help by being a fuel. The theories supporting acetone use as a fuel additive tend to be about chemical reactions, catalyzation, cleaning, or changing the combustion process...not about it as a fuel.
I don't believe it helps. I like to find ways that I can save gas, and I like to understand how to do stuff that I wouldn't even want to do, so I give it quite a bit of thought and effort.
Here's a good link busting the acetone as a fuel economy improver idea:
http://www.fuelsaving.info/acetone.htm