To a monster with 0% Cold resistance, Cold Mastery (CM) will increase the orb-damage with +5%. (Not counting the first point that gives -20% to monster cold res), where the Synergy point only adds +2%.
This is true as long as you don't push the monsters resistances below -100%.
If the monster's got some resistance to cold, the scales will tip even more in favor of CM. The higher the resistance, the more the scale tip toward CM.
as scudstorm mentioned you don't really need more than -100% from cold mastery.
most (not all) fire immune mobs have 0 cold resist. and even if they don't, it won't hurt you that much to not have them at absolute minimum.
but if you're just using fireball as your main attack you should probably max firebolt. that would increase your damage by more than fire mastery unless your +all skills is extremely low. and orb is perfectly fine as an immune killer with 1 hard point cold mastery. heck, you don't really even need to max orb itself.