The settings should be core +100 memory +300 (or +0) PL 80-90%. but even stock should be closer to 4 gps

1)Check the cpu load. Celeron can handle 2-3 1080ti max without throttling. Try disabling all but one GPUs in config.xml

      GeForce GTX 1080 Ti
      NVIDIA
      1
      0
      true


true->false or delete the whole block

2)Use command cdm -> "C:\Program Files\NVIDIA Corporation\NVSMI\nvidia-smi" dmon -s et -d 10 -o DT
under load to see if you get any pcie errors, this might be a riser issue
3)If this is a win7 machine you might want to try win10. Win7 will work but we didn't test performance.