There are thousands of threads on the subject - if you're clever enough to build a 5 GPU rig you're clever enough to use google/search function.
I have, and since I use Ubuntu 16 I have still not been able to find a way to overclock/undervolt my cards, the thing is that this cards do not have the back up bios switch.
I've tried following the guides here using atiflash/polaris using a USB stick then modding them using a WinPC..
Once I install msdos+atiflash on a USB, the atiflash does not find the cards so I cannot copy the bios so I can edit them.
I've been trying for like 3 weeks now, still have not found a good way.
atiweak/amdconfig/aticonfig doesn't work anymore.
Any help is greatly appreciated it, currently doing 22.5mhz per card