I think evaporative cooling works best for dry environment. More like desert environment it works best.
From my experience of cooling aquariums, for our rainy and humid (relatively speaking), it can cool down about 4C maybe. Another thing that will counter the effects of cooling is seeping of heat back to the system. ie. heat comes back faster than the cooler removing it. For that, we will have to see our web bulb temperature.
Do see
http://en.wikipedia.org/wiki/Evaporative_cooler, performance section if you wanna predict how low it can go. You have to calculate from dew point and humidity.
Regards
cj