I actually prefer the lower torque at the axles that the taller factory gears provide. When you gear down to 4.56 (for example) you have more torque at the wheels and I'm quite happy to avoid twisting axle shafts running 37s on factory d44s.Regearing returns engine rpms at a given speed to optimum (factory) levels, NTM reducing strain on the drivetrain.
The question isn't whether the diesel can perform without a regear; it's: can it perform optimally without a regear, and that answer is no.
As for mpg, I get 22 if I'm nice, 23 if I baby it (manual shifting and lots of neutral coasting) or 21 mpg if I just don't give a care. My commute is almost all under 55 with few stops, at mile high altitude. Pretty calm. (Driving cross country on factory 33s and no lift i got 25-27 mpg).
What's optimal for you is not optimal for me.
Moab last month on 37s and 3.73 was totally fine. Will not regear.