OK, this was a bit of a surprise. As mentioned before, we just transferred 20TB of data from a DroboPro on USB 2.0 and got a consistent 30MBps read speed. This is consistent with what Cisco says a typical USB controller can actually handle. At 480Mbps half duplex transfer rates, this is actually pretty close to the theoretic.
So now we are trying the same thing with a Thunderbolt 2 RAID 1 array (that is it is mirrored for fault tolerance. It uses a Thunderbolt 3 convertor and then is fed through a Razor Thunder 4 dock. The underlying hardware is a 20Gbps transfer rate and the drive is a 14TB disk, but it is quite fragmented and nearly full. So we are seeing speed on the order of 70-80MBps for the large files. That is a little slower than I would have expected. These are Seagate Exos X14 7200 rpm drives that are running against the 6Gbps SATA interface (so that's going to be the bigger limit than Thunderbolt 2), but the net is that the benchmarks are showing it is capable of reading and writing with BlackMagic on this pretty fragmented drive reading and writing 5GB file, we get 193MBps/195MBps read/write. The benchmarks are showing more like 225-250MBps with a 64KB block. This is sort of the best case with no fragmentation at all. For example with 50GB of small files with lots of seeking, the performance 100MBps.
Net, net 70-100 MBps reading seems a bit on the slow side, but when I listen to the drives operating, you can hear *lots* and *lots* of seeking which is very slow, so when creating these drives, make sure you do a little defragmenting if ultimate performance is needed. Also, remember this is without any striping at all, so ideally if you had a RAID10 with 8 drives, you should get 4x that performance.