i thought the solution was:
nvidia-settings -a GPUOverclockingState=1 -a GPU2DClockFreqs=300,300
It works for 3d frequencies, but doesn't for 2d.
strange enough, setting 2d clock frequencies right from the nvidia-settings gui works (it changes the lower performance mode of powermizer)
solved with this:
nvidia-settings -a [gpu:0]/GPUOverclockingState=0 #to clear some inconsistences with the gui
sleep 0.5 #just in case...
nvidia-settings -a [gpu:0]/GPUOverclockingState=1 -a [gpu:0]/GPU2DClockFreqs=300,300 -a [gpu:0]/GPU3DClockFreqs=550,900