For the "bad" performing card(s), have you tried using smaller thread concurrencies and two threads? That is, --thread-concurrency=8192, -g2, -I13?
I have two Gigabyte 7970 GV-R797OC-3GD cards and can't get the high-concurrency, single-thread, settings to work. With every version of cgminer that I've used in the 2.11.x releases, the above settings give ~750 khps.
I know this is not exactly what you were looking for, but from what I've seen, not many people are trying similar settings. I'm curious if it works for anyone else.
I've got the same card and had the same experience. 700kh/s not overclocking at all with your listed settings, I can push it higher, but maybe only to 725 or so. I'm just leaving it at stock clocks now, since it draws considerably more power for essentially the same hash rate otherwise.
Not sure why, but if you use anything other than -I 13, the hashrate takes a nose dive.