I have a GTX 1080 Ti and Core i7 8600k and no additional fans. The graphics card never gets above 60°C and neither does the CPU thanks to Dark Rock 3.

I have not installed any additional fans. The only thing I did was to remove the slots for 3.5'' disks and install the SSD on the back of the motherboard's disk. The air circulation is better that way. So far I haven't had any problems with it.

The original fans are not very quiet but ok. If the volume is not really important to you, then you don't really need one. If you install a tower cooler, you can also control the airflow with it. The circulation is more important than the number. So lay the cable nicely at the bottom and blow at the front and suck at the back. Then that's enough. It tends to be better to have overpressure in the enclosure, then less dust collects. So it might be worthwhile to have another blowing fan at the front.

It all depends on what is "not high performance" for you. As a comparison: I have two Noctua NF-A14 PWM mounted with 7700k and GTX1070: 1x at the back to the outside (support exhaust air CPU cooler), 1x at the bottom to the top (not uncontroversial; to ventilate the GraKa a bit). Cooler (Thermaltake True Spirit 140 Power) also blows to the rear. At full load I get about 78°C CPU (no guarantee, it's been a year). CPU is now just around 29°C (at less than 10% load).