I have alot of faith in the car manufacturers. they are very clever people and I'm sure that if they thought a thurmo fan worked any better they would use one. I might be wrong but I don't know of any new north south engined car that uses a thurmo fan. They use viscous fans because you don't need any fan all of the time and when they freewheel they cut down on the noise which is probably why you think your car revs better without one. Thurmos are used as an easy alternative when the radiator isn't at the end of the motor as in east west configeration. If you want to find out how much power it takes to spin a fan then look at a house or even a ceiling fan, they use very little power and only require a very small motor and they are much bigger than an engine fan. Try stopping a housold fan, it dosn't take much.
Thats what I recon anyway.
olboy