[erik]

The 40 GBit/s is definitely confusing.

My understanding is that that's total bandwidth across all protocols. So the mix of Displayport and PCIe 3.0 can't exceed 40 GBit/s. The 4x PCIe 3.0 on it's own is 32 Gbit/s, as you mentioned.

Each controller provides two Thunderbolt ports, which share bandwidth. For the 15", 4x PCIe to the left side Thunderbolt controller, 4x PCIe to the right side Thunderbolt controller, and 8x to the GPU would be a sensible configuration. Though who knows if Apple took this approach.