The graphocs cards are connected to the 16 CPU PCIe lanes that is why the x8/x8 limit, the chipset has up to 20 lanes.
However do not expect double performance http://www.pcper.com/reviews/Storage/Intel-Skylake-Z170-Rapid-Storage-Technology-Tested-PCIe...
However do not expect double performance http://www.pcper.com/reviews/Storage/Intel-Skylake-Z170-Rapid-Storage-Technology-Tested-PCIe...