I'm starting to think this lens is a bit too much of a swiss pocket knife. Too many features, too little space.
I think it would be beneficial to drop the built-in TC as mentioned earlier. It's nice as a commercial stunt, but a 200-400 without it will be more reliable and most likely sell more units, especially if it's priced to beat the Nikon version.
5D3, 5D2, Sony α6000, S90 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 70-300L, 100-400L | E-mount: SY12 f/2, Ʃ19 & 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses