Maybe a capture card down the road but then it would be 8x GPU + 8x GPU + 8x CAPTURE + 4x M.2 = 28
PCIe 3.0 on x8 mode is still VERY fast. A single PCIe 3.0 lane, at 8GT/s, can send 985MB/s. Times 8 means 7.88GB/s
That's similar to PCIe 2.0 x16.
And @lee32uk whats the deal with you and cpu bottlenecks. The 6600k would not be a bottleneck in a triple GTX 1080 setup. It's fine. If you dispute this please show me your sources.
If you are in the market for a new 1080p monitor for gaming you might want to have a look at the BenQ XL2411Z. King of the budget 144Hz budget market. Not 1440p though.
This is a bit better. This way you retain the i5-6600. http://pcpartpicker.com/list/7Zjkd6. Power supply is fine and cpu cooler is an urban legend.
About the hard drive IMO 250GB 850 evo + 2TB HDD is best compromise.