I used to have a case (Antec Super LANBOY) with the PSU mounted at the top.
I used to have a PSU with only an 80mm exhaust fan. Later I switched to a Seasonic PSU with an intake (PSU intake) fan at the bottom. I installed the PSU with the intake fan facing down.
My case exhaust fan was under the PSU like many other ATX cases.
In both cases, when under load, whenever I touched the top of my case from outside, it felt much warmer than the sides.
Considering that my case was metallic and a good thermal conductor, if the top of the case is warmer than the sides, the only thing that could cause that is the warm air. Please tell me if this is an erroneous conclusion.
Do you have any reason to believe that air would circulate at the top of the PSU at the top of the case with no fan on the top of the PSU and no fan at the top of the case (the case exhaust fan being under the PSU not over)?
Also, the case intake fan being at the bottom of the case. So, its air intake would not directly disturb the air above the PSU.
Edit:
I think your comment was with respect to my first post.
I agree 100%; warm air goes up and cold air goes down when there is no force other than gravity in play.
But, I still believe that warm air can get trapped over a PSU installed at the top of a case and installing the PSU with its intake fan at its top is not a good idea.