I have tried various values for Option "RegistryDwords" in xorg.conf but none had any effect on the Performance Mode - it only affected the Performance Levels. Also, I note that
nvidia-settings -t -q GPUCurrentPerfMode
returns 1 irrespective of whether glxgears is running. (My understanding is that Performance Mode would change depending on whether e.g. 3D was in use.) Curiously, unofficial websites have indicate 1 means "Desktop" but the nvidia-settings dialog shows "Maximum Performance".
Without knowing any way to determine what is causing the driver to select "Maximum Performance", I have to assume this is an issue, so bug report is attached.