Unfortunately it doesn't do any magic for me at 11.6 catalyst. Memory clocks can be set only 125 MHz lower than engine, as in aticonfig.
$ atitweak -m 330
Setting performance level 0 on adapter 0: memory clock 330MHz
Setting performance level 1 on adapter 0: memory clock 330MHz
Setting performance level 2 on adapter 0: memory clock 330MHz
Setting performance level 0 on adapter 1: memory clock 330MHz
Setting performance level 1 on adapter 1: memory clock 330MHz
Setting performance level 2 on adapter 1: memory clock 330MHz
Setting performance level 0 on adapter 2: memory clock 330MHz
Setting performance level 1 on adapter 2: memory clock 330MHz
Setting performance level 2 on adapter 2: memory clock 330MHz
Setting performance level 0 on adapter 3: memory clock 330MHz
Setting performance level 1 on adapter 3: memory clock 330MHz
Setting performance level 2 on adapter 3: memory clock 330MHz
$ atitweak -s
0. AMD Radeon HD 6990 (:0.0)
engine clock 940MHz, memory clock 1250MHz, core voltage 1.175VDC, performance level 2, utilization 0%
fan speed 100% (960 RPM) (user-defined)
temperature 73.5 C
1. AMD Radeon HD 6990 (:0.1)
engine clock 940MHz, memory clock 1250MHz, core voltage 1.175VDC, performance level 2, utilization 0%
unable to get fan speed
temperature 68 C
2. AMD Radeon HD 6990 (:0.2)
engine clock 940MHz, memory clock 1250MHz, core voltage 1.175VDC, performance level 2, utilization 0%
fan speed 100% (960 RPM) (user-defined)
temperature 71 C
3. AMD Radeon HD 6990 (:0.3)
engine clock 940MHz, memory clock 1250MHz, core voltage 1.175VDC, performance level 2, utilization 0%
unable to get fan speed
temperature 78 C