Sometimes it seems Schiller and Federighi are peeking over this thread

Now they're also talking about splitting the high-end Pro market with an iMac Pro.
It might be that Apple will use a nMP 6,1 approach with the iMac pro (all SSDs, ECC RAM, higher-end Polaris/mobile Vega) and go for a full size, single (or double?) upgradable GPU chassis with the 7,1.
Mago, why do you say that PCI-E v4 will take longer? Just out of curiosity.
Skylake should be out in a few months and there are many eng. samples going around. Why are you ruling out Naples? I am not necessarily an AMD fan, but there is some history between AMD and Apple (all my cards, by chance, have been AMD so far, on Macs) and it gives 8 channel memory, higher core count, higher RAM frequency - for the time being - and lower price, allowing maybe a dual GPU at the same price of the nMP 6,1.
We all (uhm.. maybe some of us!) like nVidia cards, but since right now most of Apple dGPUs are AMD, it makes sense to think about a partnership with AMD on CPUs, above all when their (AMD) marketing is pretty much focussed on Apple-style users (creatives who play a little game now and then with Ryzen, and scientific and server users with Naples).
It would help Apple differentiate from M$ who's getting a bit on Apple's feet, with their Surface range and improve energy efficiency, which Apple cares a lot about for battery and noise related issues.