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.
My blades are not running hot either. The air flow is effectively cooling the blades. They're all well under 50.
There just isn't that much heat being generated relative to the amount of air flow. I've cautioned people not to switch to "silent" (really just lower cfm) fans on S1s, but on this one I think somewhat lower capacity fans should work fine, as long as ambient is reasonably low.
You can turn down the fan while adding tunnel..... reducing volume = more air going thro what needed to be cooled.
Infact a proper design should also have a separate chamber for PSU intake.