Here's another little tip for anybody with an XR15 remote who also would really like to be able to turn everything off, including the set-top box, despite a remote design that makes this a bit of a challenge:
Issue the "Sleep now" voice command. ("Power down" also still works. These commands will now both take you into Power Preferences.)
Select "Sleep Now" and confirm the "Sleep Now" command
IMMEDIATELY press the Power button on the remote (as the STB is in the process of going to sleep.)
The Power button still sends the "Power On" command to the set-top box but it also is still on when you press the button (even though the STB is in the process of going to sleep) so the "Power On" has no effect. The STB continues to go to sleep and the other components also turn off as well.
I also do not know the difference between Settings / Labs / Power Saver Mode:Standard and Maximum Savings. Since there is "Labs" setting, I presume that the Xfinity team is working on improvements and that the "Maximum Savings" behaviour, whatever that is, will eventually be the default when "Power Saver" is set to On.