XPS 15: 9550 & 9560 Thunderbolt limitations

The PCIe lane issue only matters if you’ll be using other external Thunderbolt 3 peripherals that can use a lot of bandwidth, such as an external GPU or external PCIe/NVMe SSD in a Thunderbolt 3 enclosure. Display traffic is not carried over PCIe, so the lane count is irrelevant there. All XPS systems have dual DisplayPort 1.2 channels available over TB3, so they can handle dual displays up to 4K — and in fact dual 1080p displays can be achieved over regular USB-C with the WD15 dock. The only other traffic you’ll be sending to the dock is USB, since the built-in audio and Ethernet chips are internally connected via USB, and on a Thunderbolt dock, all USB traffic is packaged inside PCIe — but the single USB 3.1 Gen 2 interface between the PC and dock is only 10 Gbps, so 2 lanes of PCIe will be more than enough for that.

Bottom line: The PCIe lane issue will not create any limitations or bottlenecks in your use case, and in fact if you don’t ever intend to use a higher-end display configuration than dual 1920x1200 displays or a single 2560x1600, look at the WD15 to save some cash since it can handle those display setups. If on the other hand you think you might upgrade to dual 4K or even dual 1440p (a very worthy upgrade from dual 1080p that I just did myself), consider the TB16. But whichever dock you get, make sure you get the variant with the higher-wattage adapter since that’s required for properly charging an XPS 15. For a WD15, you need the 180W version. For the TB16, you need the 240W version.