I did the search but don't see a lot of info here about Toluene as a gasoline component. Here's an old thread from a Porsche forum that discusses it's use in terms of power improvement which doesn't always translate to FE, but being that Toluene is a normal aromatic component of gasoline and has about 15% more btu's/gal I was wondering if there might be some benefit.
You are correct, toluene has a slightly higher specific energy than gasoline, and therefore you could see a slight (or maybe just theoretical) increase in MPG. However unless you can get it really cheap, it is going to set you back in MP$.
I add some toluene when I go racing because it can bring my 93 octane pump gas up to 95 or 96 (depending on how much I add), and I recommend this to anyone who is pushing the limit of a street car (race cars usually need real race gas). You can add toluene up to 20 or 30% without problems, but beyond that you are diluting the other ingredients of gasoline (lubricants, stabilizers, etc.) too much and you should just go for race fuel.
I get my toluene (and xylene) from a local Sherwin-Williams store...