I've tried that and while the laptop sleeps, I wasn't able to get it to wake up again. Which of course undermines the utility of it sleeping quite a bit :-) I didn't persist too much with it, so it's quite possible that one can make it work ... but from what I read when I was trying it, it might well be the case that BIOS support for S3 sleep just isn't there.

On the latest XPS that's not even an option: https://www.dell.com/community/XPS/Ubuntu-deep-sleep-missing...

(I've just seen the last posts on that thread and will give it a try)