I've had my manual hubs since last October and now that we're finally getting the summer formulated fuel I've seen my mileage steadily increasing. I hope I end up with the 2MPG you're seeing, right now I'm a little over 1MPG increase, but I need to run a bunch of tanks through to get a good idea of the difference.
One point on maintenance, when you get ready to repack your bearings Warn sells a hub service kit with new gaskets, I think the part# is 29061. I've also heard of people going with a silicon sealer instead of the paper gasket if you run it through water since Warn recommends replacing them each time they get submerged. Also make sure to lock the front hubs every once in a while during the warmer months to keep everything lubricated.
As far as the all mode 4WD, sounds like it would be fine to install assuming you always have your hubs locked when you switch to either AWD or 4HI, otherwise your transfer case will be toast. Doesn't sound like a application for manual hubs to me.