I tested the snow mode theory for about a month, then switched back to auto. It gained me maybe 1 MPG, so it didn't help much. I live in the Los Angeles area and spend most of my time in traffic, so it doesn't matter to me whether I'm in auto, sport, or snow mode.
About a year ago, I drove to Arizona for the weekend, cruising at 70 to 90 mph on open road. The best I got in snow mode was 13.5 MPG, compared to my daily average of 11.2. I think it depends on where you live and how much traffic you deal with, from open road to congestion. For normal daily driving, the savings aren't much.
I recently moved up to 37s, and even with the bigger tires, my mileage didn't change. My advice, enjoy the truck when you can.