The 120mm PSU fan isnt designed to take warm air....
In fact the air coming out of the case (and therefore inside the case) isn't that warm. It's a lot of air flow given the power usage. Drawing the PSU from outside would probably be better but I doubt it makes that much difference.
Ofcourse its isnt warm. Thats why i say its poorly designed.
Air will find the least resistant path to go. With such a big gap between the blades, and stupidly bad heatsink, all that noise and air volume are just wasted.