MPG first reviewed the OWC Mercury Extreme Pro 6G SSD (late 2015) as a single drive (non RAID). This page assesses RAID performance in an OWC Thunderbay 4 Mini (see review of the OWC Thunderbay 4 Mini).
As this was written, OWC did not yet offer a Thunderbay 4 Mini 4 X 1TB SSD solution, but it is likely to appear and it’s trivial to build-your-own with 4 X OWC Mercury Extreme Pro 6G SSDs and an empty Thunderbay 4 Mini RAID edition (the TB4 Mini RAID edition includes SoftRAID, necessary since the demise of Apple Disk Utility).
Performance-oriented users might contemplate eliminating hard drives entirely as main storage, given this performance (except as high capacity relatively low cost backups). The graph below is over a full 4TB of capacity—it maxes-out the Thunderbolt 2 bus.
Acoustics: the OWC Thunderbay 4 Mini is now so quiet (improved fan), that MPG at first thought it had a problem powering on! An ear had to be placed right next to the unit to hear it. The 2013 Mac Pro makes a lot more fan noise. Awesome!