Just got my defective TB15 swapped out for a TB16. So far the USB connection and display connections are rock solid. Still having a problem with the USB-C port on the TB16, however. I have an Akitio SK-2520 USB 3.1 Gen 2 dual sata RAID enclosure populated with 2 x 1TB Samsung 850 Pro SSDs.
Plugged directly into the USB-C port on the 5520 itself I get the following in CrystalDiskMark:
Plugged into the TB16 I get the following:
The enclosure has external power because neither the laptop nor the TB16 can provide enough wattage to power it. It is a USB-PD certified device, so not sure why I have to use it's external power brick, but that's another discussion entirely.
At first I thought the fact that I'm driving my LG 34UM88C-P monitor (2440x1440 resolution) was eating into the bandwidth available to the neutered TB3 20Gbps connection, so I disconnected it. Even with the enclosure being the only device plugged into the TB16 the speeds remain the same. The USB-C port on the back of the TB16 has a Thunderbolt logo indicating that it's TB3 certified.
Per the Intel Thunderbolt 3 FAQ: "Thunderbolt 3 is a superset solution which includes USB 3.1 (10Gbps), and adds 40Gbps Thunderbolt and DisplayPort 1.2 from a single USB-C port." Meaning that port should be capable of providing 10Gbps of throughput to be a properly certified TB3 port.