"Barry - would you agree that the Barenboim one is a good example of the timings you like to see if one is to regard the symphony as leading us from the darkness of the Sixth to the doorstep of the 8th? If not, what would you recommend for that approach?"
Yes, I would. Another really good one in that regard is the Jonathan Nott/Bamberg S.O. one (Tudor). Markus Stenz is fast throughout, but he also has THE BEST finale of anyone. I also like the Dudamel one quite a bit.
Another good 'extreme' version is the Sinopoli. He's definitely dark and foreboding in the first two movements. He than has a pretty fast scherzo, followed by THE SLOWEST second Nacthmusik (fourth movement) of anyone. Yet, it's both mysterious and romantic sounding. It's different, that's for sure.
Zinman has a GREAT fourth movement (second Nachtmusik), and he does the centrally placed, slower 'moonlit' passage in the first movement better than anyone.
Boulez is rather extreme in that he takes 23 minutes on the first movement, yet is very fast in both Nachtmusik movements. His finale has pretty much the exact same timing as Bernstein I, Haitink I, Abbado I and Levine. You know, about 17:45
Have fun - that's the main thing. Don't make it too much like work.