I've got a 12 2500 with the 5.7 now. I've got the motor tuned with a Veraram intake and pedal commander. It's a 7,000lb tuna boat, but I think it runs pretty solid given what it is. I'd guess it would out run a comparable pickup with the 6.4 (not sure about the new ones with the 8 speed). I also have a 12 Town and Country. I enjoy driving the pickup in nearly every way (except parking lots). The van is okay. But the pickup gets lousy mileage, even if I baby it. Maybe 13 mpg on a good tank for city driving, but it's typically between 11 and 12. So it gets driven about 5,000 miles per year. The van gets driven around 20,000. I use the pickup to pull, for bad weather, and trips to the hardware store. I live where I work so I don't need a vehicle to commute.
Looking at fuelly the current 2500 with the 6.4 bge is about the same as what I'm getting now. The new 1500 on fuelly with the 5.7 is getting around 3mpg better (15.3 avg). I'd imagine the Wagoneer with the 6.4 would fall somewhere around 15 based on those numbers.