dont max Fire Mast, max firebolt if you're going for fireball. The synergy is greater
This is the point about synergies, the more synergised a tree is, the better the damage. Therefore you'll have to offset some damage in cold. Orb is still immensely powerful through NM and into hell, especialyl with about 17 soft points in CM. Icebolt adds only 2% bonus, which is tiny really, considering you can add a lot more to your fire damage with its synergies. And remember, you're mainly a fire sorc with a backup in cold.
Having a "balanced" 2 trees in sorcs is much less effective that one weighted tree.
FO's damage is mainly in sweetspotting. It can kill at a good rate with the right setup and hitting the sweetspot (I run Eilo Rytyj's Full Tal's FB/Orb (-sojs due to budget) and hit 500+ per bolt on LCS, it does fine for MFing FI areas) but Blizzard works better from my experiences with a similar setup.
Basically any sorc tele's in and statics the bosses. My use is that I give an orb to freeze it (or at least slow it down), then tele and static and then repeat until it is dead. Usually my fireball does more damage than my orb against bosses (although the Orb does very well against groups).
The order I max things is
1 in Fire Mastery
1 in Shiver Armor/Teleport/Telekinesis/Static Field
Rest in Cold Mastery