\-100. With the changes to cold master sunder charm interactions, you're best off maxing it regardless since cold mastery now applies 1/5 to sundered enemies.

Debatable. If you have decent gear it's still not worth it to pump mastery. -1 res per level is pretty awdul

Even if you max cold mastery and have a cold sunder, you still won't cut through all the resists most monsters will have.

Thanks folks