Well the thing about the W56HD is that Marlin searched and searched and searched to locate bearings that had the same inner and outter diameters but had a greater static and dynamic load rating. I think he just got lucky because this is usually impossible to do unless someone somewhere else has had some demand for stronger bearings that just happen to be in the same size as what you're looking for...
With the 'R' series transmsisions, Marlin searched and searched and searched but could not find any bearings that were stronger and a direct replacement, so we do not offer a R150FHD or a R151FHD trans.
Now if you are going to compare the W56HD to the R150F (V6 transmission) then that's like comparing apples to oranges. The R trans has a stronger housing, larger main shafts, larger output shaft, larger 1st, 2nd, 3rd, 4th, 5th, and reverse gears, larger reverse idler gear, larger countershaft size, and the real beauty of the R trans is that 5th gear, in the back of the tail housing, is supported on both sides by bearings where as all other Toyota 4WD transmissions (excluding the new Tacoma 6speed trans) only have one bearing on the front side of 5th gear. That's why 5th gear is usually the first gear to go if it is used alot. But the 5th gear in the R trans is just like any other gear, you can use it and abuse it and it'll keep on going.