Undervolt on bios with PBE isnt working on linux (or I cant do it works). This is the big problen on SM I think. +200-300watts.
When you undervolt the steps you must also use all same value example there are 6/7 steps, from 2° to higest must use same value, cause on linux TP auto select index or you can also force to use it.
Anyway use SRBPolaris and everything will be good. on rx 480 i use 975mv memory and 920mv gpu (1190mhz)