Wow.  Shame.

I threw a Corsair 1250 (non-i, just the regular not even modular version) in and I'm rocking 218 and ~1.11 Th/s VERY happily.  I left the 6th PCI-E connector off the backplane, the same as how the unit was shipped to me ... not sure if having it connected was doing anything, but when one of my cards appeared dead I disconnected it as part of my troubleshooting and everything was happy so I just left it disconnected (and after reseating and moving cards around that card came back to life thankfully) ...

Thanks for the tip about them being the same chips as the S1!  I think I saw a 225 S1 setting somewhere so I'll try the magic hex from that to see if I can get her rocking at 225 ... will report back!