Actually water wetter is a surfactant and inhibits localized boiling in the head near combustion surfaces by altering the surface tension of cooling liquids. It is "chemically" reducing bubble formation due to boiling - the same as adding fluid with a higher boiling point (ethylene glycol). A radiator cap (overall system pressure) and thermostat (increased pressure between pump and thermostat) also reduce boiling albeit from pressure-based suppression.
Reduction of localized boiling insures better heat transfer into the cooling system adjacent to combustion and exhaust areas.
And yes ethylene glycol (EG) fluids do have more viscosity than water, however, the overriding factor is the significantly lower thermal conductivity of EG vs water. I am referring to the ability to conduct heat (energy) in the scientific sense, a thermodynamic intrinsic property of the fluid. What this means is - all things being equal it is harder to make EG change temperature than water.
If your cooling system maintains the engine at the correct temperature, adding additives is unlikely to improve an already optimum thermal situation.