The case is closed, the CD-ROM works perfectly now :)... And once again the SGI support failed miserabley :(. I asked them for a copy of their firmware for that drive yet they sent me several (idiotic) steps how to check if I have CD-ROM in my system, if my mediad is working and things like that... Weird...
Well, I contacted Toshiba and they solved the opened case the same day. It happens that there are 3 models XM-6201, namely B, TM and one rare, named either B or TM (I didn't ask how to distinguish that one). Two of them share the same firmware, and the third has its own, which is incompatible with the other two. The rare drive may or may not work with the firmware of the other two (well, in my case it didn't work with the O2, but worked with the peecee).
Anywise, Toshiba support sent me two versions of the firmware, the BM1042/BM81208 (these are put in separate files but are the same actually) and the rare thing, BP71202 :). I flashed my drive with the last (this time I didn't risk and used that hardware flash memory programmer I've built long ago) and everything is fine now. I returned to the starting position, CDDA-over-SCSI have some glitches again, but they are minor to the previous problem.
BP71202 firmware might be found on
http://SIGBUS.nove.bg/pool/XM/. I've put the files they sent me from Toshiba there...