2013 Mac Pro: USB3 Performance Impaired Relative to MacBook Pro (Single PCIe Lane Throttles Bandwidth)
The USB3 implementation on the late 2013 Mac Pro is a non-starter for high performance applications because its four USB3 ports are restricted to 500MB/sec theoretical maximum speed (in each direction). Actual limits are even lower.
A RAID-0 stripe of SSDs on USB3 is no faster than a single SSD on the Mac Pro.
By comparison, the late 2013 MacBook Pro offers far higher aggregate bandwidth, and superior performance even with a single SSD.