I believe my truck has an e-fan that comes on with the AC. Then it also has the regular fan blade. My thought was not so much for extra power as it was for thinking about reducing some drag and possibly gaining a bit of MPG's ?????
Your stock efan does come on with the AC. It also comes on at a certain temp (I believe it's very high like 222 deg) so it basically wouldn't come on unless you are overheating. What many people do is remove the mechanical clutch fan and install a 180 degree thermostat, and then use a tuner to adjust the efan on temp down closer to 180 from the stock 222 or so. I think the flex fans don't work well personally....the stock clutch fan is supposed to work better actually as the clutch should release if working properly....but I don't have personal experience with the flex fans