An electric fan can save some fuel economy, but as long as your fan clutch is working properly, I'd expect to see less than a 1mpg improvement. I've seen dynos that show some larger power gains by going from an engine fan to an electric one, but take into consideration that the gains will only be at higher rpm. You aren't gaining 10-15 hp at 2500 rpm, the only time you might get anywhere close to that would be near redline. Again, if your fan clutch is working well and it isn't one of the ridiculous "heavy duty" ones that basically leave the fan spinning at higher speeds all the time, you might only see like 2000rpm fan speed when the engine is near redline. That takes some power for sure, but not a massive amount.
Biggest issue with an aftermarket electric fan is they are usually too small. Oe applications usually use 2 medium size fans (or one huge one). Aftermarket ones are usually tiny. If all you do is drive on the freeway, that'll probably be fine. But if you do a lot of stop and go or trailer towing (especially if your ac works) most aftermarket fans will be too small and you'll overheat on hot days.