The regulators I’ve used in Fat-3 and Slim-4 are rated for max 6 volts so I haven’t made a 2S version. They seem to be able to handle more, but how much more I don’t yet know, maybe I’ll test a few to death and see.
These drivers are based on a previous driver design where I used CAT4104 regulators. I’ve used CAT4104s on 4S XHP35 HI configuration with this light: Mod: BMF SRK v2 Roche Edition (Rebuilt into triple XHP 35 HI) - #18 by Mike_C
They work pretty well, but for 1S I prefer these CN571x regulators by far. They are much cheaper and handle heat much better than the CAT4104s. The 4104s have thermal shutdown, not throttling like the CN571x. It’s annoying when a few of them overheat and enter the cycle of overheating and cooling. If LED vf is much lower than battery voltage they overheat rather quickly.