SCSI over USB only really adds queuing, improving speed when many small reads/writes are performed, and you'd need an SSD supporting SCSI and an enclosure/adapter supporting SCSI over USB. Further, for large transfers plain USB 3 is just as fast, while having the benefit of being cheaper, and more readily available and compatible than SCSI over USB. Of course, straight SATA III (via eSATA if you want) is still faster.
USB 3 gets you 5 Gbps and has to be handled by the CPU.
SATA III gets you 6 Gbps without going through the CPU.
USB 3.1 promises to get you 10 Gbps (and lower overhead), but still has to go through the CPU.
And Thunderbolt is just a convoluted and expensive way of piping a limited number of PCIe lanes to a random physical port and requiring the user to buy an expensive cable. 10 Gbps or 20 Gbps. 40 Gbps in the next revision.
SATA Express / M.2 can get you 32 Gbps using 4 PCIe 3.0 lanes or 2 PCIe 4.0 lanes wrapped up in NVMe.
And you can always just throw more PCIe lanes at some controller (on-board or an via a PCIe slot) or some device directly if you want more bandwidth.
USB 3 will be the standard for external shit for a long time. The C connector and USB 3.1 are going to have a hell of a time gaining traction.
For people who want performance, SATA Express / M.2 using NVMe or other direct PCIe solutions win.
