One thing that both of you seem to have ignored with respect to the taller tires is frontal surface area. If you put a taller tire on, the frontal area goes up by half the diameter increase x the section width. So unless you lower the truck to compensate for the increased frontal area, the results will be skewed against what gain it might otherwise produce.
I have just learned one nice thing about our 4WD: It has a 2" lift block installed over the rear axle which when removed levels the truck out.
I'm going to remove them and see what reducing that rear tire frontal area does. 2 x 2" x 10" = 40 sq in. that will be tucked up further into the wheel well. True, it's barely more than 1/4 of a square foot of reduction of exposed drag surface. Hopefully it will reveal some benefit however. (I'd argue it's in addition to frontal area, but that's probably not a technically correct way to define it.)
Regardless, I don't think my shell concept is going to happen for awhile if at all. I just don't have the time for it with all of my other projects going on. Most likely we'll just get a folding hard tonneau for now.