its done all the time . say its 90 degrees out side why do people use fans lol it cools the air be low ambient air temp i'm not blowing 90 d air on myself. as far as this application go's water cools better than air . its more efficient.
If it is 90 degrees outside and you have a fan blowing at you, you are blowing 90 degree air into your face. Where your face differs from a piece of steel is that your body uses a system of perspiration to promote evaporation. The increase of airflow over your body increases the rate of evaporation on your skin which leads to lowering skin temperatures. That's trait of humans, not heat exchangers on cars. I.e., -20 degree (F) day in the winter with a -60 (f) wind chill feels like -60 to our skin, but it's only -20 as far as the radiator on your car is concerned.
and a fan running on the heat ex you can cool it a few degrees lower than air temp
That will never happen. The heat from the fluid is transferred to the heat exchanger which transfers the heat to the ambient air as it is flowing through the heat exchanger (either with the help of a fan or not). Once the fluid inside the heat exchanger reaches the same temperature of the ambient air, no more heat transfer will take place.
heat transfer occurs between two regions of differing temperature. Once both regions are at the same temperature there is no excess heat to transfer. Thus you can't get below ambient air temp. If you think you are, then your temp gauge is wrong.
Most Air To air intercoolers also have a fan, either the one on the radiator, or their own fan, which helps to cut down on stop and go driving heat soak, or recovery after a race.
water always cools better than air.
Water has greater heat capacity than air, but it can't get rid of any more heat than the exchange medium will take. As long as you are counting for ambient air to take the heat from the water, you'll never get the water temp below ambient air temp. Water weter works to reduce the surface tension of water so that more water is in contact with the surface of the heat exchanger, allowing it to pickup more heat from the intercooler and make more heat available at the heat exchanger.
Bell intercoolers has a good faq page:
http://www.bellintercoolers.com/pages/techFAQ.html
This wiki article may help you put into context the physics behind moving heat around.
http://en.wikipedia.org/wiki/Convection
http://en.wikipedia.org/wiki/Conduction_(heat)