Was thinking about this talent and it made me wonder, why dont you turn it into a dot that ticks over the the 5 seconds duration? That will make the spell a lot more useful in PVP and also in M+.
Currently SR is useless in PVP and useful sometimes in high M+ keys.
Imo they should just make it a passive for all 3 specs. Makes obliterate, scourge, and whatever the blood version would be to do X% damage additional shadowfrost dot to targets below 35% hp. Dot can’t be refreshed but can have damage added. Once it falls off then it can reapplied. (If target dies while affected by the dot, it jumps to the next lowest hp target in combat and refreshes duration)