I have made it through. Indeed, some risers turned out to be bad and were messing up the entire thing. I managed to get all 6 cards running under Windows 8.1 + Catalyst 3.12 + hashcat 1.01 (later Catalyst didn't want to start up; I also didn't WindowsUpdate) and under Ubuntu 12.04 using epixoip's wiki page. Windows setup was quite easy, just install the OS, then install Catalyst and then all GPUs are detected with no problem. Ubuntu was also fine given the wiki page (it's worth noting that after changing GPU configuration - number, slots, etc. - one should reboot into single user mode and mount -o remount,rw / && amdconfig --adapter=all --initial -f - otherwise X will hang; Windows does that automatically).
Then I've set up VNC server with x11vnc - it's better than others because it captures X framebuffer directly instead of creating new X session which eliminates configuration of OpenCL running under VNC's session (I couldn't easily do this with tightvnc, for example). Given epixoip's autologin setup it becomes essentially a standalone headless box.
Also, I discovered that my screen didn't like DVI input from any of the Sapphires (resolution above 1200 would be all messy) so I had to use HDMI.
Then I've set up VNC server with x11vnc - it's better than others because it captures X framebuffer directly instead of creating new X session which eliminates configuration of OpenCL running under VNC's session (I couldn't easily do this with tightvnc, for example). Given epixoip's autologin setup it becomes essentially a standalone headless box.
Also, I discovered that my screen didn't like DVI input from any of the Sapphires (resolution above 1200 would be all messy) so I had to use HDMI.