Using spellfire is a supernatural ability. Are there any feats to increase its damage output? Materials from Dungeon and Dragon magazines are welcome.
Only a handful of feats even modify supernatural abilities, and most of those feats won't increase the damage of the supernatural ability spellfire. The feat Empower Supernatural Ability (Tome of Magic 74) is probably the best candidate, and it's tragically only usable once per day. Further, while that feat's description mentions the possibility of maximizing a supernatural ability, the feat Maximize Supernatural Ability remains apocryphal. Presumably one could talk the DM into allowing something like the feat Weapon Specialization (Player's Handbook 102) and picking the supernatural ability spellfire, but this reader doesn't advise a player spend his PC's feats that way.
In other words, if a creature really wants to deal more damage with the supernatural ability spellfire, the creature should increase its Constitution score like with the epic feat Great Constitution (Epic Level Handbook 56).