Have you seen anything much from birders saying the 1DX III isn't as good as a BIF rig? It is a bit odd, but I have seen remarkably little on it in terms of true BIF evaluations. But, Jonathan is doing pretty well with it
I look for this frequently and there isn't much that I can find. Jonathan has shared some wonderful results....his owl images are wonderful. The relative lack of user reports is one reason I'm not completely convinced about the 1DXIII being my ultimate BIF solution.