As an Amazon Associate I earn from qualifying purchases @AMAZON
As Photoshop Scratch Disk for diglloydHuge Benchmark
OWC Mercury Aura SSD for Mac Pro: 1TB about $899, 2TB about $1479.
The diglloydHuge benchmark was used to test Photoshop speed when a fast scratch disk is needed. The benchmark eats up ~56GB of memory. Since the 6-core 3.5 GHz 2013 Mac Pro had only 16GB memory, this forces constant disk I/O as the test runs.
As can be seen, the Aura Pro is neck and neck with a triple RAID-0 stripe in the OWC Thunderbay Mini, even though in theory that configuration is much faster.
It can also be inferred from this graph that a 4-drive stripe or the Apple flash drive would be little different from what the Aura SSD delivers.
Higher raw speed does not always translate to higher real-world speed, because applications have their own bottlenecks. See notes that follow.

OWC Aura SSD for 2013 Mac Pro vs triple RAID-9 stripe
Photoshop throttles performance

Photoshop CC 2014 writes in 1MB chunks *at most*. If a smaller “tile size” is used, then the transfer size might be as small as 128K.
It is why a Photoshop scratch faster than ~600 MB/sec helps, but only marginally; the relatively small transfer size of 1MB does not allow peak speeds to be reached. MPG has made certain recommendations to Adobe on this front. The whole tile size thing is an anachronism that ought to be removed, with Photoshop dealing in 8MB or 16MB chunks.