DPReview has measured the point that Sony a7S III switches from its default, high dynamic range readout circuit to lower read noise (and lower DR) readout method for higher ISOs. Since the ISO rating depends on the tone/gamma curve, the switch in gain depends on which Picture Profile you use. Standard Picture Profiles make this move at ISO 2000, while S-Log profiles will see the gain at ISO 16,000.
This means that with standard picture profiles, a7S III offer less noise at ISO 2000 than ISO 1600 – but also less dynamic range. S-Log cinema profiles experience a similar shift between ISO 12,800 to 16,000.