From what I see in lspci, the WD15 appears to be a true 4-lane Thunderbolt dock along with 4-lanes of DisplayPort 1.2. Which is what the user manual suggests it is, contrary to everything else I have seen written about it. On the XPS 9360, it is limited by the "half bandwidth" USB-C port, but on a more capable box (eg. 9370) one might reasonably expect it to do even better. TBD.

I did find this very nice explanation of how a USB-C connector's bandwidth can be divided up among USB2, USB3, DP, and PCIe:

https://www.dell.com/community/Laptops-G...ock/m-p/5153006

EDIT: The USB-C cable of the WD15 has a proprietary "docking connector" at the dock end. I wonder if this cable is the source of limitation/confusion, and perhaps the dock itself is capable of more if it had a different cable plugged in?

Yeah, that must be it. In the user manual, it talks about capabilities with a "universal cable" versus a "USB Type-C" cable. So perhaps Dell only equips it with a restrictive form of "USB Type-C" cable rather than the "universal cable" that could do full ThunderBolt. I wonder if there's a replacement part number somewhere for that "universal cable" ?

But then, they might also restrict things within the dock firmware as well I suppose.