One thing that I found interesting about flexible vs. rigid diaphragms is that the rigid diaphragm analysis creates torsion on the structure and basically adds the additional load into the other three walls. You basically eliminate the third wall (portal frame wall) entirely. So why so much fuss over portal frames, make the diaphragm stiffer, treat it as a rigid analysis and assume the portal frame wall basically does nothing in resisting the lateral load. Breyer's book has a good treatment on this, looks like I need to do some more reading...
The prescriptive bet is the safe bet, I agree. However, I would like to be able to actually engineer one of these with some confidence. If I increase the strap capacities, holdowns, anchor bolts and sheath both sides of the portal frame with 15/32 Structural I Ply/OSB where does that put me? I should be able to get a little more than the prescriptive IBC/IRC portal frame. Consideration for the concrete foundation stiffness and the header stiffness also factor into the design as well.