About Fire vs. Shadow in 2.4...
The new 0/21/40:
Talent Calculator - World of Warcraft
Watch out!
That being said, the differences between Fire/Shadow in 2.4 are these:
Shadow Bolt:
+10% from Shadow Weaving
+20% from ISB (depends on how often it's up)
85.7% of +damage Coefficient before Shadow & Flame.
572 average damage base
Incinerate:
+15% from Scorch
+10% damage from Emberstorm
+10% haste from Emberstorm
71.4% of +damage Coefficient before Shadow & Flame.
598.5 average damage base (with Immolate)
Overall, they're strikingly similar. Shadow Bolt gets 5% less via SW than Incinerate gets from Scorch, but makes up for it with the higher +damage coefficient. Incinerate gets a flat 20% DPS increase via Emberstorm, but Shadow Bolt gets almost 20% from ISB (since it's not
always up), but helps the raid too and does not require Immolate (which with all the fire talents, might actually be higher DPCT anyway for an Incinerate build).
As far as personal DPS, they will compete. As far as raid DPS, 1 or 2 Shadow Destruction Warlocks are necessary to keep ISB up 50-60% of the time. Additional Warlocks doing that won't really help that much since more often than not, they will be replacing each other's charges and adding minimal uptime (observe this effect via
WOW ISB Uptime Simulator).
Imagine you have infinite Destruction Warlocks with 30% crit and 10% haste (about right for BT/Hyjal). Imagine you have two fixed Shadow Priests (again, about right for BT/Hyjal).
1 Warlock: 50.6% ISB uptime (50.6% increase over 0, 10.12% shadow DPS increase)
2 Warlocks: 59.07% ISB uptime (8.47% increase, 1.69% shadow DPS increase over 1 Warlock)
3 Warlocks: 64.1% ISB uptime (5.03% increase, 1.01% shadow DPS increase over 2 Warlocks)
Notice that with each Warlock you add, the ISB uptime increase is smaller. 1 Warlock responsible for ISB is the largest effect. Each additional Warlock past 1 Shadow adds a minute amount of ISB uptime and % increase to Shadow DPS. This effect is caused by overwriting ISB's; the more Warlocks you have doing it, the more ISB charges you will be overwriting. A single Warlock maximizes ISB usage and efficiency.
Bottom line: The ideal is likely going to be 1 or 2 Shadow Destruction, the rest Fire.