I have 18 LifePO4 cells, all from AW.
I have 2 of them set aside to treat badly, just to see.
My latest torture was dragging one down to 0.67v last night.
It soon bounced back, charged, then powered a light (Romisen RC-N3) fully for testing, just like they always do, same runtime, no apparent difference.
I'm not suggesting anyone intentionally abuse batteries, but if you're going to, LifePO4 are the ones to do it with. They seem even more robust than expected. These 2 that I have been treating badly, I've been so mean to them that I certainly wouldn't put them in a 2 cell light along with a non-abused cell, just in case, but for single use lights they appear to be just as good as when I got them.
You do have to consider the long term effects, but occasional dipping below 2v, or even lower, seems unlikely to have a negative effect on a LifePO4.
For the record, I also have an E1B on the way (my first small Surefire), so I know what will be running it