Even if it managed to get 80% at 500w in the real world (much higher ambient temps than 80plus test requires), thats still 100w of heat to get rid of, plus whatever is coming from the rest of the componants if you have a convention case with the PSU at the top.
I think it would really struggle to keep within a reasonable temperature with that much heat and no active cooling. I would be very uncomfortable at putting it inside a case where it might draw any more than 300 watts. Its chassis suggests it might want to sit on top of your case more than in it.
One review i found (here
) tested it in open air in a 20c room. Really, what the hell is the point in that? Put the thing inside a case (which cuts off most of the ventiliation holes) and crank the heat up. Can it cope with 35c+ intake with much less ventilation? Because thats how it will have to run in the real world.
EDIT: Oh ok, i noticed this NorthQ Pacific FANLESS
power supply has a fan, which under the reviewers highly unrealistic test kicked in around 75% load (on the 400w version they tested, so about 300w). I would expect it to kick in at around 200-250w in a normal case, which is well above idling load, but still, far from great.