I have a 2010 Mac Pro with a 8-core, and I'm in the process of hunting down some x5690's.
My SSD drives are really old. Maybe the first batch of Intel SSDs. Is there a point of doing a RAID array of these old SSD's? It seems like it would be smarter to put everything, scratch, catalog, and pictures that are being edited on the new 1TB Apple SSD.
Retire the Intel's to system backup images.
Have you ever studied
http://www.macperformanceguide.com tips on optimizing for Photoshop etc?
When I saw another thread, and using only standard SSD's like Samsung 850 - whether EVO or Pro - instead of PCIe-Express blades which can be 3.5x faster (but you still want enough memory so a lot of work that was done by scratch and cache disk storage can be done in memory).
If you had something from Samsung 830 era, Crucial or SATA III units, then "maybe" give them something to do.
Your 5,1 means that slots 3&4 are not tied together to share bandwidth and hopefully fewer conflicts when using 2nd GPU or a 3rd party graphic cards or PCIe USB3 or SATA III controllers - and can even run a stripe raid using two adapters or controllers.
The old way to get Apple blade level of performance was:
Two SATA III SSD controllers and 4 x SSDs of 250GB or 500GB.
Such contortions are history now - like running 2 or more Ultra320 SCSI controllers and 8 x 15K drives were once, or 10K Raptors later, in order to get fast scratch. Fast being relative and 800MB/sec was considered very good. A single 500GB blade can easily do.