This product is definitely not worth it for multiple reasons:
1) Performance is sub par
2) Sandforce solutions from OWC tend to run hot especially in tight situations where cooling is difficult
3) The Mac Pro was built with external expandability in mind, 1TB internally should in all reasonability be enough really.
4) Price. Put together 2 Samsung 2TB 850 EVO's in an USB3 or Thunderbolt enclosure, configure RAID0 and done, faster, less expensive and just a tiny bit less convenient.
The only thing speaking in favour of this SSD is internal placement. For all the times you want to take your Pro with you and nothing else..