true, i think ISO 6400 in some scenarios. i think for sport, high ISO is needed more as lots of sport are played in dim lights/indoors.
sometimes 1/1000 is not fast enough tbh and if you want less shallow depth of field to help focus tracking your going to need to sacrifice the ISO
Sports is very different - you normally have a shorter, faster lens, need faster shutter speeds (1/1000-1/4000) and are often shooting in poorer lighting. Live music or stage performances are even more challenging in terms of low-light capability, but you aren't focal length limited. No doubt the 5Ds is suboptimal for these kinds of photography.
But, at the moment, I'd take a D810 over a 1Dx any day for shooting wildlife; the 5Ds looks even better, since it combines high resolution and top-level AF with the ability to use the Canon 200-400L with its inbuilt 1.4x TC.