All on the same pickup I had:
4.10's with N/A stock tires
4.10's with Turbo stock tires
3.55's turbo stock tires
I have currently
3.55's with turbo and the large tires @ 285/75-16.
MPG's never really changed much. I averaged about 15 mpgs, except with the large tires it's about 14mpgs. The trick with these large tires is lowering the air pressure to get even treadwear. Most people don't do this, but I do and I love the ride on these softer tires. They just float over everything and road bumps are nothing. Love that.
I think I'm at 30psi front and 20 rear for DD'ing. Even those pressures still wear out the center faster than the shoulders.
The problem is the better the turbo setup, the more you use it.
So even though I would get better MPGs especially cruising with stock tires, I don't because driving faster and more tire drag.