Cool burb! I lust a little for a newer burb with a cummins but it would be hard to make that work as clean as the dmax swap. It's good to see some realistic milage numbers coming up too, I get tired of big tire trucks that get 20mpg towing at 80mph. I think you would see a noticable increase in milage swapping back to a stock bumper, I put a ARB bumper on my '03 GM dually and could tell immediately that it's an aerodynamic nightmare. Milage is still good at lower speeds but interstate driving milage got pulled down for sure. Nice bumper and I don't know how to put good protection up front that is aerodynamic so it kind of just is what it is. I had a friend nail a big doe (mule deer, so big comparatively) on the interstate and just had to re-align the ARB bumper on a GM like ours. Without bumper it would have been pretty ugly.
My truck is a all around MPG whore. With the lift, weight, and bumpers I don't stand a chance. I have been driving it all over and the best I can get is almost 18 mpg. I love to drive it and it is very functional for my purposes. There are big deer all around where I live, it's amazing how much damage they can do to a truck.