So, just like the 22 Gbps cap on PCIe speeds that Thunderbolt 3 has, I found that Thunderbolt 2 similarly has an 11 Gbps cap on PCIe transfer speed. I found this illustrated in this Intel slide:
What that means is that an external NVMe SSD drive connected to Thunderbolt 2 would only be about 10% faster than being connected to a USB 3.1 Gen 2 port (10 Gbps).
I was not able to find any comparable Intel information on Thunderbolt 1, but I assume it must have a similar data transfer cap. 5.5 Gbps?