Cant be donde whiout increasing voltage, voltages cant be increased beyond bios limit in Linux, AMDOverdriveCtrl only allow voltages changes in bios limit (so nothing, as the limit is the stock voltage).
I just noticed that
voltage adjustment with Sapphire 5850 Xtreme works. However, power consumption rises about 80 W/h, even with slightest changes, e.g. 1,088 V -> 1,094 V. Two of these cards are in my PC. Temperatures rise over 80 °C, too.
I use Catalyst 11.6 and SDK 2.4 with phoenix.