Wayne,
I have a body kit on my Infiniti Q45. It's by IMPUL. It drops the line 2 inches all around. But I got it because I wanted it and befor I became an eco-modder.
With that said, I do think the kit helped w/ my mileage prior to going Eco Mod because I was getting 25-26mpg on the freeway and the epa is 22.
The other advantage was it made doing underpanels easier because the 2" lower cleared EVERYTHING under the car!
I think there have been discussions about needing to be 'so close' to the ground to have an effect.
Also I had lowered the front end this last year 1.25" and got some mpg gain there. Not sure if you would get any gain on a pickup truck with that.