Using three OWC Thunderbay 4 units, I explored the performance boundaries if I/O. Each Thunderbay unit was connected to one of the Thunderbolt 2 busses on the 2013 Mac Pro so that each would have full TB2 bandwidth.
The result show near linear scalability through 8 drives, with 9 drives and beyond dropping slightly in performance. This is a lot of bandwidth and could be useful when (for example) cloning from one RAID to another.
Note that more than 4 drives in a RAID-0 is increasingly risky because the volume is lost by failure of one drive—use RAID-5 for fault tolerance (OWC offers a Thunderbay 4 solution with RAID-5 preconfigured). But this test was about exploring the performance playground on the 2013 Mac Pro.
Click for larger graph.