You can't load new firmware on the consumer units (notice the absence of the white switch we have on the dev units) you can only update the controller firmware. The current firmware I have only used for scrypt but will try dualmining when I get home. I understand it works but didn't look into it. The current software it will work but not show you the output you need to look at your pool. The devices draw 7 watts in scrypt mode by to cpuminer on the computer doesn't have the ability to turn off the power to the sha cores that is why you are experiencing the high power usage. I don't think the controller has that issue.
Just to make sure I understand, the firmware that provides the 500khash rate from a single unit is for the controller, not the device itself? So the devices that the rest of us get aren't going to be capable of any more than 300khash when connected to PC?
Cpuminer has the option to clock at 800 the same as the controller so yes you will be able to achieve the same hashrate. I have stably clocked up to 900 on devices fine.