My two cents rant about PSU fan facing up or down when it is mounted in the bottom of the case when talking about semi fan less power supplies.
Heat goes up. When PSU fan is off, all that little heat will go up directly to the GPU.
When PSU fan is on, you are cooling PSU with a warmer air from inside of the case.
When PSU fan is off. Heat goes up. But between PSU circuit board and GPU you have PSU metal case acting as heat sink, so in theory it should not heat GPU so much. But, in theory, heat should accumulator faster in PSU case itself and PSU fans should start spinning sooner.
When PSU fan is on. You are taking cooler air from outside that should cool PSU faster. Considering PC case has some gap from the floor and air intake holes.
And plus. If You have water cooling and it is mounted somewhere above, and one day it fails, in this case You probably wont have water dripping through Your supplies fan hole directly in to the high voltage circuitry, at least immediately…
My verdict – bottom mount power supply should be facing fan down.
In real life the difference should be minuscule if we talk about full or mid size PC cases. Some tests on the web show that fan facing down makes PSU few degrees cooler. Some show no difference at all. So decision can be made even by overall aesthetics i.e. in what position everything looks better.