Look into your Xorg.log. Maybe your cards pixel clock is the limit there (tho I don't think so actually)
Doesn't matter if the monitors support it if your video device doesn't.
However, as I said, I don't think this is it.
Just go to your ccsm -> general options --> display settings
and disable "detect refresh rate".
I posted some troubleshooting
This thread is 2 years old. You bet nvidia would have long released a fix if it was related to the Powermizer.
However, you can give it a try. You don't need any additional scripts neither, PowerMizer may be configured inside your xorg.conf too. Just read the manual nvidia provides (which is VERY informative actually)
You could try and enable CoolBits for troubleshooting purpose too... No need to state that you'd do this at your own risk I think.