for a pure frozen orb sorc, is it beneficial to max out cold mastery? since the Sunder breaks immunity to only 95%

Isn't it true that maxing out cold mastery is kinda useless as you can't go past -100% resistance?

they just changed cold mastery to work at only 1/5 effectiveness vs broken immunes (similar to conviction and lower resist), so maxing it gives a huge dmg boost against this group of monsters