just put together my first mining rig (3*6950) and was playing around to get reasonable MHps rates out of it.
I'm mining at deepbit with guiminer and started first using the gfx cards untouched. I got some 270-280 each. Increasing core from 800 to 880 with MSI afterburner moves the performance to the 310-320 range, giving an overall performance of ~950 MHps (which is also reported as my rate at deepbit).
Now I'd try to tweak further to get the kickin' numbers from the mining HW comparison list that gives a max of ~365 for locked HD6950s. Alas, I don't see which measure to use for reliably detecting MHps.
Increasing core or voltage values further with afterburner, I see the numbers displayed by guiminer (which in fact are poclbm values) fluctuating between 180 and 320 with no clear trend indicating an increased performance. After a while (~3 min) the values stabilize near the old level of ~320 (but still fluctuating between ~290-340). As an effect, it's impossible to reliably determine if the last tweak was helpful.
Even more strange are the values reported by deepbit (averaged over last 5 minutes): every now and then the MHps rate varies quite significantly in a range between 700 and 1000 with sporadic outliers going down to 300 and peaking up to 1150. And this really confuses me: how can the mean value reach 1150 while guiminer was not reporting values > 950 the previous 10 minutes?
I might be missing something, but the observed issues indicate (at least for me) that there is a lack of reliable MHps measures. My best guess so far is just listening to the fans and correlate noise and performance
Or, how do you measure your MHps values?