There is more to it than just the iops.

@50K iops you are consuming > 1/3 of the CPU on the AMD rig, the 9260 has it's own "cpu" and shouldn't be consuming > 10% in most cases.
As the QD increases, CPU utilization on the AMD (and Intel's as well) steadily increases and in the end they'll consume most of the processing power available on the computer, this comes at a cost. (other tasks suffer)

OTOH, there was never any doubt about the onboard controller being competitive for most desktop tasks, just look at the ICH, a race-horse at heart and the SB850 is indeed a great step up from previous chipsets.