UPDATE 2:
Ran a test side by side with 2011 MacBook Pro, both running identical Premiere versions, Using same external T3 storage and project file / media. Playback the exact same clips. etc etc…
2011 MacBook Pro outperforms my new 2016 MacBook Pro in playback.
will take laptop back to Apple see where we go from there.
Hopefully I don’t fall into a black hole between Apple and Adobe in where the problem lies.