The plot thickens... I decided to see what would happen if I shrank the Windows partition by 40 GB - and voila, the "accelerate" option appeared again. To make things even more interesting, it remained available even after I'd resized the partition back to its full size...
Anyway, I then switched acceleration on, using enhanced mode and the whole SSD. Now Windows does not even start, not even in repair mode. Looks like I'll have to tear the RAID array down - and given this laptop doesn't seem to let one enter RAID config at boot time, I will have to do this the hard way i.e. manually from a Linux rescue CD.
In order to recover acceleration option, you needed to wipe off SSD performing low level format, this will clean metadata and any structure previously created. You should see acceleration button enabled again.
That worked for me. To "low level format," I did
select disk # (0 for me)
Then I restarted the RST program and the accelerate tab was there.
(My PC immediately became unresponsive when I enabled acceleration though. Mouse moves but Ctrl-Alt-Del doesn't work. I won't hijack this thread with that though.)
It is also important to mention that the OEM systems are always flashed with a specific image that has been designed and manipulated by the system manufacturer. This image also includes all pre-installed software in the computer with all the configurations that it requires.
If after doing the low level format on the SSD the system started very slow, I recommend you contacting the system manufacturer, in your case Lenovo, so that they can provide a copy of the operating system that was installed in your system.
Their contact information is the following:
Unfortunately the suggestion to low-level format the SSD was irrelevant here - if you read my first post carefully you will find I *have* in fact wiped it, to no avail.
Anyway, from what I can see the problem which prevented SRT from being enabled was that additional free space was required on the HDD at the end of the disc (or possibly after the Windows partition - couldn't tell the difference with how the disc in question was partitioned). The original partition layout, the one for which SRT did not work, had the Windows partition stretching all the way to the end of the disc (I used gpt-fdisk under Linux to create that partition, as I used a Linux LiveCD to wipe the discs); on the other hand when I told Windows to shrink its partition and then expand it back to full size it still left 1 MB of unallocated space. The issue looks fully reproducible at this point: if I somehow allocate that last 1 MB the option to enable acceleration disappears, when I make that space available again it reappears.
The problem with Windows initially not booting after I'd got SRT going again was most likely related to a hardware fault of the SSD - a few days ago it stopped working altogether. In other words, it was almost certainly not relevant to the matter of enabling acceleration itself.