EDIT: Looks like some of my bugs were caused by HWiNFO running. Having HWiNFO running and changing voltages and/or timings in Wattman triggers some weird memory bandwidth bug. I have to do more tests to be certain.
The bugs concerning the ~210 GBytes/s to ~193 GBytes/s and reverting voltages still stand.
Tested hardware: Sapphire Nitro+ RX580 4GB / Windows 10 64bit (19.5.2. and 19.8.1. drivers)
ASUS P8P67 LE / i5 2500
Tool used: poclmembench (GitHub - kruzer/poclmembench: calculates your gpu memory speed)
This profile does not survive a computer's sleep mode. Even when I stopped using the sleep mode, sometimes the profile still gets bugged. My custom profile is stable, this still happens even if I increase the voltages slightly above default values.
This brings me to memory timings and memory bandwidth.
Number 1. is my normal, default memory bandwidth/speed. Number 2. is a bug that happens when I change a voltage of any power state (it doesn't matter if I increase or decrease the voltage).
This isn't a bug in the poclmembench; slight decrease in performance can be observed in games. Restarting the PC is the only solution I know of to get the full ~173 GByte/s speed back, keeping your custom voltages as well.
Let's jump to memory OC.
When I OC my memory from 1750MHz to 2000MHz and change memory timings to level 1, I should get ~210 GByte/s, but this doesn't work most of the times. Often changing memory timings does not improve performance at all, due to a bug in Wattman. It doesn't matter if you have your timings set to automatic, level 1 or level 2; bandwidth won't change. This doesn't happen all the time, often you will get a bandwidth increase, but it's lower than it should be. Sometimes I only get ~183 GByte/s with the same settings when I should get ~210 GByte/s.
When the OC works as intended and I get my ~210 GByte/s, after restarting the PC the OC is broken and I get ~195 GByte/s. As shown on this screenshot:
I believe this is not intended at all. Way back with older driver (I can't remember the version anymore) I was able to keep my ~210 GByte/s OC even after PC restart/shutdown, but now it gets completely broken and goes down to ~195 GByte/s.
This nasty bug happened when I hit the "Reset" button in Wattman to revert to default settings (once I even got ~53 GByte/s and performance was very low in games):
As you see memory speed went completely down and gaming performance was affected quite a bit... Again I had to restart my PC to get the default ~173 GByte/s.
You may not have these bugs, but I do on my RX580.
Drivers were reinstalled using DDU as well, I even disabled Windows 10's fast startup feature as recommended on AMD forum.
I hope you can test this as well and report this to AMD too.