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

I got paid 2 times, 3 LTC total. still have 2.4LTC these, but today I cannot even log in. Yesterday I have noticed that my wallet address is incorrect. Current hashrate these is 9.5 Mh/s, so 99% miners left.

these is PN: 299-1E249-100SA , no idea if these are 4L or 2L, This type has 2 big fans not one as yours. Core is set on 1200 mV or otherwise it was not very stable.
As measured it is pulling average of 1250W~1300W out of my 1200W PSU
Cards are pretty close, as there is not much space between (few mm)
I have extremely big fans in case cover to put more air inside

so it is only 200W difference, it might be ok I think.
I assume that is around ~300W per card on my setup, ~350W on your setup...

getting 610 khas/s with this on Saphhire 7950:
--scrypt -I 20 -w 256 --lookup-gap 2 --thread-concurrency 24000 -g 1  --gpu-engine 1100 --gpu-memclock 1500

setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
cgminer xxxxxxxxxxxxxxxxxx --scrypt -I 20 -w 256 --lookup-gap 2 --thread-concurrency 24000 -g 1  --gpu-engine 1100 --gpu-memclock 1500 --auto-fan


This one was getting around 610 khash/s on Sapphire HD7950 Boost 3GB 384Bit

Hi

I have similar setup. Also using sapphire 7950. I have Athlon X4 750K CPU, on some Asus MB. To be honest I did not try to measure consumption yet. My only difference in config is that I am running at 1100/1500 all cards, getting 610 khash/s per card. My temperatures are around 75C on GPU's.

I will connect Wattmeter and let you know, but since I have only 1200W PSU, it could not be near to yours