Did you try with the latest drivers?
Catalyst 11.6 seems to have the restriction removed (you don't even need amdoverdrivectrl)
Adapter 0 - AMD Radeon HD 6990
Core (MHz) Memory (MHz)
Current Clocks : 955 1250
Current Peak : 955 800
Configurable Peak Range : [500-1200] [1250-1500]
GPU load : 0%
Adapter 1 - AMD Radeon HD 6990
Core (MHz) Memory (MHz)
Current Clocks : 955 1250
Current Peak : 955 800
Configurable Peak Range : [500-1200] [1250-1500]
GPU load : 0%
you need amdoverdrivectrl i think.. look at your current clock and current pleak:)