In my experience the 1dx III outperformed it's EV by a bit. I never did an exhaustive comparison to see it's limit. But needless to say the lighting was so low you couldn't see very well it was so dark. This was on the Sigma 105mm f/1.4, which has quite good AF along with a wide aperture. So that always helps.

As we all know it's only partly indicative of AF performance, but with that said. -7.5 EV is indeed very impressive!