It doesn’t use PCIe gen3 x16... the GPU interfaces are completely custom called MPX and that is before we get to the Afterburner card which I suspect is Apples first foray into using their own silicon in a desktop...Who wants to drop a tonne of coin on PCIe gen 3? Single Socket... etc etc.
4 slot cooler though.
At least it has DDR 4. Welcome to 2014.