Adding to this thread, I found the following:
https://thunderbolttechnology.net/sites/default/files/HBD16235_Thunderbolt_TB_r05.pdf
It is fairly detailed, but two paragraphs stood out:
"The dual-port SKU supports two Thunderbolt 3 connectors. It takes four lanes of PCI Express Gen 3 as an input
and two full (four-lane) links of DisplayPort 1.2a. "
-> which implies that the dual port controller can handle 4 x 8Gbps + 2 x (4 * 5.4Gbps), which with the protocol overhead removed equates to c. 80Gbps, so maybe it is 40Gbps
per connector.
"Because a Thunderbolt 3 chip can support either one or two connectors, there is the need to provide more capability than can be used on a single connector. Many Thunderbolt 3 usages are around single-connector consolidation, but there are also many consumers who want huge expansion with di erent devices on each port. "
--> which also may imply that a single connector may get less bandwidth the combined capacity of the controller (obviously!)
The diagram in figure 4. is also interesting, and might explain the OPs view from the System Report. The physical connector seems to actually have two bi-directional data streams of 20Gbps, so a combined logical bi-directional stream of 40Gbps (i.e. in each direction).
*But*, the MBP 15 would presumably have
four of these "port pairs", and the OP is only showing one. Is this correct?
[Edit: apparently the reported bus speed, e.g. 20Gbps, can vary according to the connected devices. If a TB2 device is connected, it may only report "up to 20Gbps" even if the port will support 40Gbps, as only a one of the two 20Gbps bi-directional channels is used (per port). Plugging in a TB3 device may change the reported bus speed]