Thanks for your thoughts, HNx.
The controller is a HighPoint 2720SGL dedicated hardware RAID controller, though rather a lower-end model that doesn't embody its own cache. It can easily sustain virtually the full SATA III bandwidth on all four ports at once, and has been an utterly reliable device.
I originally formatted the array during Windows 7 installation and the partition configuration is optimal.
Notably the Highpoint driver does not support TRIM operations, but this doesn't matter. Without TRIM the SSDs manage their own free storage erasure via their internal Garbage Collection algorithms, and these really work. Over the 1.5 years I used this array with Windows 7 my benchmarks and practical I/O experiences actually got faster and faster as time went on. That has reversed with Windows 8.1.
The SSD experts on the OCZ forum confirmed that a lot of people reported long-term speedups, and it has to do with the in-drive SandForce controllers getting familiar with usage patterns and organizing things to be most efficient. Part of succeeding at using SSDs long term is maintaining a sufficiently large amount of free space - counterintuitive given the hardware cost, but it really works.
My observation, since moving up to Windows 8.1, is that most I/O operations have been getting incrementally slower and slower. Not terribly much so in the general case - from applications I still get about 1.5 gigabytes/second throughput (though it was 1.8 GB/sec under Windows 7), and a few operations faster than ever. File Explorer is an exception, it's much less efficient. Everything I can measure implies the drives are just fine - it's the Windows software that's becoming less efficient.
No one can say whether it's on purpose. And apparently not everyone sees the same performance. Some folks report they can enumerate tens of thousands of files per second in Explorer, though it's always possible they are still running Windows 8(.0) which was fast.
There is clearly something very specific being done in Windows 8.1's File Explorer that TREMENDOUSLY slows down operations, as compared to prior versions (e.g., Windows 8's File Explorer). I say watch what they pull out of their, er, hat with the file system implementations in Windows 9 - I'll bet Microsoft magically somehow makes it fast again, and they'll exclaim what heroes they are for speeding things up so amazingly.