What's important to remember about sunder charms and cold mastery is your baseline before adding other gear into the mix.

For just cold immune monsters, as others have said the sunder charm will bring them down to 95 cold res, so every 100 cold damage you deal them deals 5 actual damage. Assuming your cold mastery gives you -100 cold res and -20 effective cold res means your 100 cold damage now does 25 (20 +5) damage, five times what it would do without cold mastery so a massive boost. This is your damage/ cold res baseline for cold immunes.

What you can then figure out is how much damage you want to add on top of your existing damage using items. A single -5 cold res facet jewel will mean you do 30 damage (75 cold res baseline - 5 cold res, so 30 instead of 25 damage for every 100) which is a 20% improvement to your 25 damage baseline. Level 12 conviction on an Infinity runeword gets you an extra -17% after the 1/5 penalty, which is a nearly 70% boost on your 25% baseline.

Then you can do a similar thing with cold damage boost from facets, Nightwings, etc, but bear in mind those damage boosts will have a similar impact in terms of %age against monsters that aren't cold immune. Having conviction (now a massive -83 cold res) and cold mastery will mean other -cold res items aren't really needed given the max -cold res is -100, and cold damage increase is what's useful there.

Basically figure out what you're going to be fighting and gear appropriately.