I have it at 20% more power in cataclyst also... that's overvolting i believe.
Do you think you could check whether changing the Powertune in catalyst drivers actually impacts your hashing speed or power consumption?
I have a few EAH6950 cards myself and although the driver reports success, the powertune value has no effect whatsoever on any of them.
As to how one overclocks on linux, along with the catalyst driver you installed a little utility called aticonfig which is able to change gpu/memory clocks or fan speed.
Alternatively, do as DeathAndTaxes suggested: cgminer is a GREAT miner (possibly the best?) and it integrates mining, overclocking and fan control in one neat package.