I saw that episode too and the Mythbusters test was anything but comprehensive or representative of real life. I've been there, done the acetone thing, and didn't even get a lousy t-shirt, let alone better fuel economy. Actually, my economy didn't change at all, so buying the extra acetone actually made my cost-per-tank higher. I did, however, gain 5mpg by changing and being more conscious of my driving....and that was free.
The only purpose acetone serves in fuel is to better keep premix oil suspended in methanol.