Toluene and xylene will not mix with water, so you will have two separate liquids. Also you will lose the extra benefit of water's cooling properties, and you will destroy the pump in short order. Thats why we never put more than a 1:4 xylene/gas ratio in the fuel tank.
With what you are suggesting, injecting straight C16 would have the same octane, but there would not be enough of it to cool the charge properly