Thanks mate. So I guess thats not that big help, you are just scartching fire imm mobs.
By itself, only Cold Sunder on Sorc with Cold mastery seems like a good Sunder. I havent looked into this too much, but yeah if you dont add other -res it's pretty much useless.
Before ROTW I tried the magic sunder on Hdin and it was absolutely ridiculous, Smite being more effective than Hammer with Sunder. There was no -magic res whatsoever so it was basically useless. Now it has changed.
If you can manage to get -fire res gear it should prove efficiency but without it yeah it is useless, unless your fire dmg is so high that even 5% of it is enough to clear, which even with high fire dmg I doubt.