Hmm, while the adapter looks good, I eventually just opted for shoving the SSD in hard drive bay 1, and moving the drive that was already there into my second optical bay. The cost of a 5.25" to 3.5" adapter, a SATA cable and a SATA power cable (from 4-pin power) didn't cost more than £20. For 2009 Mac Pros onwards you should only need the bay adapter since the connections are all there already I think?
The main issue I had was getting the SSD onto the hard-drive tray, as the screw spacing for 2.5" drives seems to be just that tiny bit different, so the tray wouldn't slide all the way in (only by one or two millimetres, but enough to stop the side panel locking). So I ended up having to spring an extra £8 for some thermal tape to hold the SSD to tray; a bit haphazard looking but a 2.5" SSD is so light that I half expect the SATA connection would hold there, but I don't really want to test that theory!
Anyway, this gives me a completely bootable setup that can also see the SSD used as a Bootcamp volume, bypassing the need for a PCIe card, as while it would have been neater overall, and I haven't really used my second optical drive enough to justify wanting to keep it inside, so I just moved my Blu Ray drive into optical bay 1 and now use that for everything. I have plenty of hard-drive space so I can just create a disk image in place of copying from one drive to the other 🙂