I shoot birds-in-flight (BIF) with my 1DX2/EOS 600mm F4 lll and believe I have a good grasp of the AF custom case settings, as well as BIF shooting techniques. That said, the 1DX2 uses "predictive" algorithms that causes the subject tracking to be unstable against backgrounds with contrast. The AF of the D6 and A92 are biased toward tracking and maintaining focus lock, once acquired. You mention Ari Hazeghi. Ari is a highly accomplished BIF photographer who shot Canon for a decade. Last year he sold his Canon gear and now shoots the Nikon D6. Ari has written in detail of the shortcomings of the Canon AF for BIF photography. Another dedicated BIF photographer is Art Morris. Art was a Canon Explorer of Light but now shoots the Sony A92. Art is outspoken about the limitations of the Canon AF for BIF photography. I was recently shooting BIF with Art in Bosque Del Apache. The tracking of the A92 is remarkable.
Canon said in a recent interview that improvements in AF tracking is one of the main developments of the 1DX3. If true, I am a buyer. Canon also said to expect a reduction of weight with the 1DX3 which is great news! Another big improvement will be adoption of dual CFexpress cards and adoption of HEIF files. I am curious to see the IQ of the compressed HEIF files, and the benefit, if any, HEIF may have on FR and/or buffer capacity.
BTW My daughter is a wedding photographer and is ecstatic about her results with Eye AF for still photography.