To summarize: Go with the 80%, screw the Sunnis.
It is rather ironic, when you think about it. The U.S. is stuck in a COIN effort, mainly against Sunnis.
Counter-insurgency being what it is, the U.S. is of course attempting to win over the Sunnis, and U.S. efforts are probably preventing an all-out civil war, complete with ethnic cleansing that would leave the heavily outnumbered Sunnis in a distinct pickle.
Thus, the U.S. is insulating the Sunnis (again, the primary source of current American woes in Iraq) from the worst consequences of their own failure to maintain control over the country after decades of repressive rule over the others; namely, extermination, or something close to it.