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The D800's AF is supposed to be "f/8 capable", so it should be able to reliably use autofocus with lenses as dim as f/8. The 80-400, at the longer end, is an f/5.6 lens, and with a 1.4x TC (a 1-stop light loss), it will be an f/8 lens. So in other words, AF with a 1.7 TC (a 1.5-stop light loss), it may not AF when the max aperture is smaller than f/8.
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