SCSI -> SATA seems like an unnecessary step when SCSI adapters are available.
Disregard people who want you to do complicated things. It is very rarely, pragmatically speaking, worth it. SCSI to SATA is a very expensive way to have exactly the same experience as you would with cheaper options.
I'm really at a loss to understand how 10MB/s is just as good as 20MB/s, and that $100 for a SATA adapter and SSD is too expensive, but the same spent on SCSI2SD or SCSI adapter + Ultra-wide SCSI HDD is a smart spend. Granted, the Apple cache+SCSI PDS card is very difficult to find and likely expensive, but it is grail-worthy, and I highly doubt anyone who's ever posted here with a Q900, Q950 or WGS95 would not deeply consider the expensive purchase and have all sorts of desire for it, if available. Consider that if I spent twice as much for 20MB/s as you did for 10MB/s, we paid the same in $/MB/s. But it is too complicated for mortals, so do not consider it, and dismiss all dissent! Seriously? It isn't splitting the atom, folks, it's a SCSI card, a couple of SCSI adapters and an SSD, and the appetite to make it hunt for the glory of twice the performance.