Underolting in linux works only when you set lower powerstage but this works not always !
For sure it's working for my rig but with very limited range

Or when you have custom bios with addedd undervolting offset

I would avoid additional BIOS modding except what is required to get 30MH/s.
Perfect solution for SMOS would be to have additional two fields (OC page) related to mem/core voltage but as I understand it's not possible. Right?