At what max temps should I keep my R9 390s?
At what temps do you keep them?
Anyone has any good BIOS mod for them?
in general is the rule, low temps are prolonging the lifespan of the components.... (heat kills them..)
so try to get them as cold as you are possible, in the cost efficient way...
my temps are between 52°C - 71°C ...(fans between 70%-75%)
try to make the best airflow in the rig, and place the GPUs properly.. idk how much space u have..

With 52c and 70% fan speed, you will be replacing a lot of fans with no gain in longevity.
Use box fans and try to turn down the gpu fans to 40-50%.
Anything under 80c will work for years.