100-400 II IQ with a 1.4X is acceptable, though not great, and only allows AF at the center point. Of course I can get about the same IQ from the 200-400 with a 2X on a 1DX compared to 100-400 + 1.4x on a 70D/7D II, but there is no net reach gain. For most people a DX body with an...