Maximum PC

The 2011 Dream Machine included the Z68, and thus SRT was available. I understand that they used a pair of SSDs as the boot drive, and thus those SSDs were unavailable for caching. The more typical caching case involves using the mechanical drive as the boot drive, and then caching on the secondary SSD. But the Dream Machine included 3 mechanical drives as well. So couldn't they have added one more SSD and used that to cache the mechanical drives? Would that be a valid configuration, or is it for some reason illegal? It would seem desirable to get more speed out of the mechanical drives, even if they're not the boot drive. (As a practical matter, they might have run out of SATA plugs, but that could have been remedied.) I'm asking because I was thinking of building a similar system, that is, boot from SSD with a secondary mechanical drive, and was thinking it would be good to cache the hard drive by adding one more minimal SSD devoted to that purpose.

It would be pointless. SRT only really helps with the boot drive, and when the boot drive is already both RAID and SSD, you are already on maximum performance, at least that's how I understand it.

yup... SSD Cache isn't going to help / would actually make things worse with SSD raid as the boot drive

It stands to reason that SSD caching would be an improvement for any structure, boot drive or not. The principle is fundamental: You map the gigabytes of the cache onto selected portions of the disk, and then accesses that would be slow become fast. This can be useful as long as some portion of the disk gets relatively heavy access. So I don't think you can expect the non-boot SSD caching to be other than good, although, depending upon usage, the improvement may not be significant.