2-Way GTX 1080 uses 32 PCI-E lanes. Add a 950 Pro and that takes up 4 more. It's not marketing.
Do you have a review showing more than 1-2% benefit at high gaming resolutions of running 1080 SLI on 16x/16x over PCIe 16x/8x or PCIe 8x/8x?
Until there is proof that shows otherwise, it's marketing.
https://www.techpowerup.com/reviews/AMD/R9_Fury_X_PCI-Express_Scaling/18.html
The irony here is that i7 6700K max overclocked paired with DDR 4000 and 1080 SLI would beat any i7 6950X (and any other Broadwell-W chip) in games as far as minimum and average FPS are concerned. So you trying to justify 1-2% PCI 16x/16x delta and ignoring Skylake's superior performance in the first place only undermines your argument.
Z170 boards can easily do SLI and the fastest Intel drive too.
It's now possible to build a workstation PC and the fastest gaming PC in the same case.
http://www.phanteks.com/Enthoo-MiniXL-DS.html
Why is it "price inelastic" PC gamers don't buy the fastest workstation consumer PC and the fastest gaming PC and put it into the same case?
Fact is i7 6700K OC with fast DDR4 will beat BW-E in games. Broadwell-E is the best all around Intel processor but not the best gaming CPU due to 1 generation behind architecture (unless it can overclock beyond 5Ghz). If 6950X maxed out at 4.5Ghz, the gap between that and i7 6700K 4.7-4.8Ghz is even greater. Without many DX12 games, the 8-10 core BW-E cannot utilize that power which means it won't even be faster for games than the 6-core BW-E parts.
AnandTech's sample couldn't even go beyond 4.1Ghz:
http://www.anandtech.com/show/10337/the-intel-broadwell-e-review-core-i7-6950x-6900k-6850k-and-6800k-tested-up-to-10-cores/10
That means there is a real chance of getting a slower $1723 gaming CPU in the 6950X than a stock $310 6700K.