The latest tweet from Phil Spencer reveals that he is already enjoying a working XBox Scarlett prototype, so it looks like the specs are almost finalized. Spencer also confirms that the upcoming console is backwards compatible with existing game titles and network infrastructure, plus it supports accessories like the Elite Series 2 controller.
https://www.notebookcheck.net/Microsoft-s-Xbox-team-leader-Phil-Spencer-is-already-playing-on-the-Xbox-Scarlett-next-gen-console.446015.0.html