OEM PSU's are generally the last things you want to use.
I'd be surprised if you got 80% efficiency on it. That would put you at 240W of peak performance.
Efficiency is based off of conversion from AC to DC, it doesn't affect the rating on the sticker. 80% efficiency while using 300w DC would mean you're pulling 375w from the wall.
You're approaching it's limit and wondering why the fan is on all the time?
And then you want to manually turn the fan down?
I'd never let you near my PC