Here is an update.
The new replacement device was fine (it displayed excellent sleep performance) until the installation of build 1511 and related firmware updates (of course forced by Windows Update) . Now the sleep study produces the same type of behavior as the exchanged
device.
I expected that the issues would be back given probable causes. Tech support at the Microsoft Store was fully aware of this issue and had dealt with other situations like mine (replacements). When I exchanged the device they stressed the disclaimer that
the new device may indeed have the same issue as the old one. When pressed, they acknowledged the firmware is likely impacting ALL devices equally. It's just that many users are not aware of the actual sleep issue in technical terms, e.g. are not tech savvy
enough to run a sleep study.
I had tried to stop all Windows Updates and put up with the pre-1511 version. But Windows Update "owns" are devices in a way that forces a user to not be able to protect their device when they know an update mucks it up.
The Surface Book owners are in the same boat based on their comments on the SB forums.