You will get an increase ONLY when the fan is OFF. IF you were able to replace the mechanical fan with an electric capable of doing exactly the same amount of work, then you would actually lose when the fan is on because you would be converting mechanical (belt) energy to electric to mechanical again and lose in heat dissipated in the alternator, regulator, leads, and fan motor. However, this would mostly happen idling in traffic. Above a certain speed the fan does very little, and the radiator gets most of it's cooling from the induced air from the vehicle moving. At these speeds a mechanically coupled fan (for example, with the fluid clutch siezed) would be a net loss of power. I think that the power you would gain wouldn't be noticable at the pedal though. About the same difference as driving with lights/defrost/wipers on or off.
