I totally agree with your post Shiner, and like I said before, it's because of all the weight of the components, drag and other things that bring down the fuel mileage.
I have a friend that could build a BT4 to put out well over 350 HP. But, it will not have to duty cycle of the 6.7's or the 6.6. To keep the duty cycle up you need big heavy components. The 6.7 Cummins in a light duty pickup will not have the duty cycle of a Cat 400 HP 3208. The 3208 will weigh a whole bunch more than the Cummins, but the Cummins will burn a lot less fuel too because of the weight of the components.
It's a give and take world out there.
