Maybe a tenth, my motor is more responsive without logging a huge fan 24/7 for no reason, free wheeling or not. Made 0 difference in my alternator because it hardly ever runs, just when I stop in intersections and it's adjustable, in or out, switched so I can manually engage or I can run it in the staging lanes to cool things down while waiting for the burn box.
One huge 14" fan keeps my cammed 350 between 160 and 180 all day on the hottest days, one of the best mods I even did was taking that stupid mech fan and huge drumlike wind tunnel cave off and running a steamline Durale. I can reach everything without the mech dumbell. It takes HP to turn a fan on the end of your cam, mine runs cooler and seems to spin quicker. All the new stuff is going electric.
I would recommend it just for the instant response and options vrs the old style power robbing one direction on all the time junk from the factory.