I recently build quad 7950 mining rig. all Saphire cards clocked to 1100/1500.
When I started cgminer I was getting 640kh/s, until temp risen to 70C, then it slowly dropped to 580kh/s per card.
So I removed 3 cards and let only one for further investigation.

I was not able to figure out why it has so low mining speed until I accidentally installed program called HWinfo64 (http://www.hwinfo.com/)
As I run the program I noticed that while memory clock were at 1500MHz, GPU clocks were changing from 1005 to 1050 every second.
After some trial and error I found that when I increase voltage on GPU (--gpu-engine 1100 --gpu-memclock 1500 --gpu-powertune 7) with cgminer powertune or in catalyst control panel to 7% or more GPU clocks were stable at 1100, not fluctuating anymore.

From now on my quad setup is running 640kh/s on each card total 2660 kh/s

maybe this can help some of you increasing hashing rates
Of course temp was going up a bit, now I have around 76C per card, before it was 74C