Not sure but if you can detail what port you are testing I can try when I get home later today. I just got the board last night so I will be tinkering from Friday evening into Saturday. We can see if the bugs we encounter are common among the Maximus...
Mine arrives on Thursday because NewEgg hates me. On a side note a review we put up. Reading it now.http://www.guru3d.com/articles-pages/asus-z170-rog-maximus-viii-extreme-review,1.html
The 950 is M.2. If the M.2 port is listed at PCIE 3x4 which means PCI Express Generation 3 at 4 lanes you will get the full speed of the 950 pro. I plan on getting one myself.