I recently added gages to my truck to read boost, back pressure, vacuum on the wastegate and fuel pressure. I just returned from a trip to the FL Keys and did some experimenting on the trip. One interesting thing surprised me. I normally pull about 4k with about 2k in the truck bed and drive 70 on the interstate. I am geared to run about 2050 at this speed. The back pressure is always higher than the boost by at least 2 psi. Stock boost settings gave level road readings of 3-4psi boost and 5-6 psi back pressure. I got on all tanks of fuel 14.5 to 15 mpg except the following.

On one tank I adjusted my homemade boost fooler to get a about 5-6 psi boost which gave 8-10 psi back pressure. This tank I got 14 mpg. The worst on the trip.

I also removed the vacuum hose from the waste gate to see how much back pressure I had just from the open exhaust and it was less than 1psi.

I think a more efficient turbo would help mpg. My gages have both been calibrated at 5 psi so I know the readings are pretty accurate.

One other thing the vacuum gage shows about 7" of vacuum under stock boost at my driving conditions. With more throttle or idle it goes up significantly.

These mileage figures are the best I've ever got. On similar trips with the truck stock I got about 11 mpg.