It's official now.
Single drive on AHCI, XP 32bit, P55, no freeze, no error events with 9.6 so far (had problems with 8.9), but this need some time testing.
However, the problem is still mentioned in the "Known Issues" section of the Release Notes:
Event ID 9 "iaStor" error in the system log
Windows XP, Vista
One other thing, it seems they forgot to change some of the F6Readme contents:
The Software contains pre-release "alpha" or "beta" code, which may not be fully functional...
Best version yet, and I confirm that sound no longer stutters using this rev.
Asus Rampage Extreme mobo w/ICH9R & 6 x 1.5tb disk in raid 10 (1.4TB mbr C:, 2.8TB gpt D:, 15GB mbr P: (for pagefile.sys)).
I can also tell you that ATTO benchmark shows the same read/write performance characteristics comparing the mbr to the gpt volumes, and this was not the case for any previous version of IRST.