Methanol vapor may be dangerous to inhale. you should take it into account.
Methanol vaporizes faster than water so that it abosorbs more heat than water in that temprature.
Water has bigger latent heat of vap but if you spray more methanol you can eleminate this advantage of water.
I recommed MTBE for increasing octane.
Cheaper is ethanol but it should be pure and i advise not more than 15 percent. Iso octane is expensive to use increasing octane and have less octane rating than MTBE and ETBE. in additon the oxygen content of ETBE and MTBE advantage for prformance of engine.