Besides the fire risk, there is also the issue of overshoot. The swings are too much to handle for the PID system and you end up with noticeable oscillation of nozzle heat. It 'can' work but will stress the mosfets and generally be less than optimal so it just isn't worth it. I complained to the ebay seller and just got my money back.
Oh, I think that the PWM figure will end up around 20% as the a 40W 12V cartridge will put out 160W at 24V.