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My cards mine at 620 khs, they can do better but my PSU is only 1100W so I wanted to stay around 1000W consumption to get the best PSU efficiency. So I lowered my voltage a lot. It's strange cause you should be able to easily reach 600 khs with your 7950.
Thanks for feedback. Cracked it yesterday, i read on a forum a point about -g (lookup gap) value being 2 is wrong. I've always used this for whatever reason, so i re-read the cgminer readme and it says don't touch it, its relevant only to low Intensity settings. It dawns on me that this increases something at the expense of intensity, but with the trade off of a usable system - at I=13 you wouldn't know the machine is hashing even for 500-550 kh/s. With -g 2 i could go above I=13 or 14 with out spewing HW errors. With -g 1 i can happily set to I=20. now with threads pushed higher too im pulling 630 kh/s woot!.
Lesson to OP: read lots of forums (try searching card type/name + mining), read the damn documentation

and experiment, keeping records of changes and their effects.