If you're going to go fire, go with this build:
Talent Calculator - World of Warcraft
You should ALWAYS take the necessary talents to do maximum DPS with BOTH Shadow and Fire.
Needed for shadow:
ISB.
Needed for fire:
Emberstorm, improved immolate.
The rest of the talents apply to both specs. Ruin, Shadow and Flame, Devastation.
So hmmm if you only need ISB to still do maximum DPS with shadow, why does everyone take cataclysm? It's like they think there's some kind of hidden rule that if you're a "firespec" lock you aren't allowed to use shadow spells.
Cataclysm? It's useless. It's purely a filler talent. The only reason people take that talent in the first place is because there's no other places to put the talent points. After spamming 50 nukes I save the mana cost of 1 lifetap. wuuuuuw. That's going to make a difference.
If you take ISB, at least you have the option of going shadow if necessary. Oh no. This mob is immune to fire *cough, flame of azzinoth on illidan* damn, I don't have ISB.
By the looks of it it's going to be "necessary" for you to go shadow the majority of the time. Why go fire spec without a fire mage? To lose 15% damage?
Also, backlash. Why would you NOT want 3% crit to all spells? If you didn't notice it gives 3% crit, check the talent :p
If you're relying on soul leech to keep you alive in bosses like BT, your healers suck.