Check your submited shares for me will you? That thing I saw was that they just didnt perform that well with 6 on that setup.
That hashrate is computed from the submitted shares over a ~three week period. I don't trust the miner figures either: I've certainly seen cases where turning the overclock up lowered the share rate while raising the hashrate without showing any errors.
To be fair, I had some stability problems while I was running 2x950 PSU which went away when I switched to 950+1200 and switched to 240v (I didn't both at the same time so I don't know which to credit— the PSUs I tested _do_ manage to handle more load at 240v). I've also noticed reduced performance from more GPUs but I found this was addressed by increasing the intensity and worksize.
(and without increasing stale rate in any meaningful way— .... now I'm mining using p2pool, which has a 10s merged blockchain for payout agreement, and my stale rate on it is much lower than typical)