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?
no more speed is only better if the watt cost is worth it.
I can get 660 on a 7970 but watts will be over 270.
same piece of gear down clocked to 500 say .90 volts/ 880 core 1.5 volts 685 memory will burn only 140- 150 watts. so yeah 680 sounds good compared to 500 but not if i pull 270 watts vs 145 watts. now my watt cost is 17 cents a k watt. lots of are people lower then me so oc may make sense.
In the winter I got a shit load of free heat so I oc'd and my winter heating bill went from 230 usd to 130 usd. now the summer oc make no sense as my ac bill jumped so I underclock