There's 4 40Gbit/s ports, but their total bandwidth is not 160Gbit/s. IIRC, the ports on the right side of the machine have less total bandwidth because of how they're connected, for example.
That is correct. The Thunderbolt controller has two DisplayPort sink ports which are wired to the GPU, in addition to the PCIe 3.0 4x interface which goes to a root port in the CPU. So to actually max out the 40 Gbit/s, you need to drive external displays and also max out the PCIe 3.0 4x interface. Which is another way to say that 8 Gbit/s are reserved for DisplayPort.