Do MB before SWP. Although SWP does more instantaneous DPS than Mind Blast, SWP has a very long duration, losing half a tick by casting MB first does not result in as much of a damage loss than delaying MB with it's relatively short 7 second effective cooldown.

Also in the notes at the bottom you stated that Inner Focus + DP is 3 seconds, Inner focus is an instant, not an instant cast, meaning it does not invoke the GCD. The same goes for potions, healthstones, and similar items but NOT Shadowfiend.

From my calculations the slightly lower DPS/cast time of SWD combined with the 71% (or more depending on haste) longer cooldown reduces the oportunity cost of delaying SWD enough to warrant casting SWP or VT before SWD.

From a mathematical standpoint, casting MB before SWP cannot be beneficial if SWP deals more damage per cast than MB. This is fairly simple to prove.

Let's say you have two players, Alice and Bob.
Alice prioritizes SWPain over Mind Blast. When SWP wears off just as MB comes off cooldown, she chooses to SWP.
Bob prioritizes Mind Blast over SWPain. When SWP wears off just as MB comes off cooldown, he chooses to MB.

Other than these two spells, their casting priorities are the same. They therefore will cast exactly the same number of Mind Flays, Vampiric Touches, SWDeaths, and Vampiric Embraces. The only difference is that Alice casts more SWPains and Bob casts more Mind Blasts.

These two shadowpriests will eventually get to the point where they are casting a Mindblast at exactly the same time, but Bob has cast one more Mind Blast than Alice over the duration of the fight. What did Alice do in those 1.5 seconds (minus haste)? Because Bob and Alice cast exactly the same number of "other spells", this means that Alice has cast one additional SWPain.

Therefore, as long as one SWPain deals more damage than one Mind Blast, you should always prioritize SWP over MB.

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This can also be proven with a much shorter logical arguement.

The oppurtunity cost of one Mind Blast is 1.5 seconds, modified by haste.
The oppurtunity cost of one SWPain is 1.5 seconds, modified by haste.

Since they have the same oppurtunity cost, you should cast whichever one deals more damage-per-cast.

This is starkly different from the Warlock argument, where the oppurtunity cost of Curse of Doom is 1.5 seconds per minute (modified by haste) while the oppurtunity cost of Curse of Agony is ~3.75 seconds (averaged).

Re: Tymir

I've spent much time over the last two days thinking about this problem (lol) and it's an interesting thought experiment. I've come up with at least 6 wildly different ways to look at the problem. I suppose it's all moot with 3.0 as SWP will refresh itself, but I am morbidly fascinated at this point.

I don't believe it's fair to consider Mind Blast as a 6second cooldown in this scenario. Mind Blast and SW:P can conflict with each other no more frequently than every 24 seconds (the duration of SWP). Whether you Mind Blast every
5.5 seconds or every 8 seconds, you can cause a delay in Mind Blast no more than once every 24 seconds.

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Consider the ways in which you can refresh SWP:
1) SWP falls off, MB is still on cooldown. You cast SWP. Your MB cooldown is unchanged, and you lose no SWP uptime.
2) SWP falls off, MB just came off cooldown. You cast SWP and then MB. Your MB cooldown is delayed 1.5sec.
3) SWP falls off, MB just came off cooldown. You cast MB and then SWP. You have lost 1.5sec of SWP uptime, and your next SWP will come 1.5sec late.

We will ignore situation #1 because it doesn't affect anyone. Let's again consider Alice (SWP first) and Bob (MB first). Let's say that over a certain period of time, both Alice and Bob have encountered 5 SWP/MB conflicts:

Alice: MB's cooldown has been delayed by 7.5 seconds. SWP has not lost any casting time.
Bob: MB has not lost any casting time. SW:P has lost 7.5sec of uptime.

Woohoo, you say - Bob has gained 2,900 damage from that extra Mindblast, while Alice has lost 2.5 ticks of SW:P for 1600 damage. It's good to prioritize MB over SW:P!

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Stop. Reality check.

Bob's total casting time is 7.5 seconds longer because he has let SWP fall off for 7.5 seconds! Alice's casting rotation takes less time because she can reapply SWP more frequently. We are effectively comparing a 24 second cast rotation to a 25.5 second cast rotation - apples and oranges! It is misleading to directly compare the two.

The truth is even trickier than this. The fact is, most of the time there will be no conflict between SW:P and MB and both players will refresh SW:P exactly the moment it falls off. (this is a thought experiment, so both of them can have 0msec effective lag. Maybe I should rename them Cameron and Cromartie.) Sometimes Alice will have a conflict and Bob won't - Alice will fall further behind in MB cooldown but Bob will not lose any SWP uptime. Sometimes Bob will conflict and fall behind, while Alice has no conflict and loses nothing.

If you look at a graph of total damage dealt, at every point from 1.5sec to 24sec, Bob will appear to be 1.5 seconds ahead of Alice. However, Alice gets to refresh SWP 1.5 seconds earlier. If you consider a DPS cycle as starting by refreshing SWP, Bob's DPS cycle is 1.5 seconds slower than Alice's. This is true if the conflict happens right away (24sec vs 25.5sec rotation), but also if the conflict takes several full cast rotations. (ie 72sec vs 73.5sec rotation)

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The Answer.

There is only one point at which Alice and Bob can be directly compared. This is after Bob has delayed SW:P by 24 full seconds - that's 16 SWP/MB conflicts. At this point, Bob is ready to cast his Nth SW:P. His SW:P timer is 24 seconds behind Alice. This means that Alice is ready to cast her (N+1)th SW:P.

How many additional Mind Blasts has Bob gained over Alice?
The answer is surprising: Exactly one.

Other than MB and SW:P, Bob and Alice cast exactly the same number of other spells. Therefore, if Alice has cast one additional SW:P, Bob will have cast one additional MB. No matter how improbable this seems, it is logically certain.

What happened the 24 seconds shorter cooldown on Mind Blast? It's an illusion. Bob purchased a 24 second shorter Mind Blast cooldown at the expense of spending 24 seconds longer to cast his spells. In the end he only comes out one Mind Blast ahead, and it doesn't matter what the cooldown of MB is .

Don't believe me? Run a sim or better yet, construct a simple Excel spreadsheet. You will find I am 100% correct.

I made a dead-simple spreadsheet where you can only cast two spells (Mind Blast and SWPain) and time is divided into 1.5second quanta. There are two columns, one which prioritizes MB and one which prioritizes SWP. You can plug in any values for Mind Blast cooldown and SWP duration and you will get one of three scenarios:
1) The "MB first" rotation is grossly more efficient than SW:P first. (less conflicts)
2) The "SWP first" rotation is grossly more efficient than MB first.
3) The rotations are roughly equal, and for every extra MB you get, you lose a full-duration SWP.

What's interesting is that scenario #3 is what happens under "realistic" conditions of 24sec SWP and 6sec MB CD. #1 and #2 are unrealistic in any case, because your filler spell rotation is unpredictable, and combined with lag this will tend to equalize the overall number of conflicts.