My take is it further increases the density. Remember that the more fuel you can burn the more power you can make. if you add nitrous + methanol you are adding more air and fuel to the combustion making it more dense creating more power.
No matter if your using nitrous or boost. you are adding more air to the engine. so how can anyone say there would be a benefit to adding water meth inj. to a boosted engine but not one with nitrous?