OK, finally got it working. As posted here https://superuser.com/a/691690/248151 IRST requires some free space at the end of the drive, so previous time when I tried that it didn't work, because I had other volumes after C: ... shrink the last volume and you'll get acceleration options in Performance tab.
It reduced my free space with 2MB and I was able to extend the last volume back after initially shrinking it with 64MB