Sunders set cold immune monsters to 95% cold resist.
Cold mastery then applies at 1/5 effectiveness. A -200 cold mastery is actually -40 for sundered monsters.
For example 95 monster res -40 from cold mastery = 55 monster res. Then 55 monster res - 53 from doom =2 monster res.
I always max cold mastery, but it’s a personal preference. Also if you go blizz, go all in and work on its synergies before orb.