(Another reason why the 1DX overlap is smaller than you think: 1DX users aren't looking for 90-95% as good as something -- they want the best money can buy in that format. That separates the users pretty clearly for me.)
Very true. The 5d3 is 95% of the 1Dx and it is half the cost. Once you get that good it is really expensive to get just a little better.
I'd put it more at 75%. I've shot extensively with both, indoor and outdoor sports, and with regards to AF accuracy and metering accuracy, 1Dx is winning by a fairly substantial margin. In terms of AF accuracy I'd still put the 1D4 ahead of the 5D3, but not in ISO performance. That and all the other things Marsu42 listed...