With my 7950's, I can get 590 on each @ 1120mhz and its stable.
Under the tune and tweak options if I uncheck manual vectors it increases hash speed considerably. I haven't checked that against completed work units to see if more are actually being done, but I assume the increase in hash speed by unchecking manual vectors is better?
I have 2 kinds of 7950, the 3 of the Sapphire 3L's and one sapphire single fan model. The 3L's have a switch on them to run at 1.169v or 1.25v. I run them at 1.169 and get about 640Mhash. Another machine with the exact same hardware only gets 615Mhash. They both are xubuntu 12.4 and the only difference is one has Catalyst 12.8 drivers and the slower one has 13.4 drivers. Besides that they are identical.
/edit I'm running cgminer 3.1.1 on both.
The other cheaper 7950 is locked at 1.25v an gets 630Mhash.
I've managed to get all three cards up into the 675 range, but they aren't stable running that hard. Only last a few hours before they hard crash the machines.