On a less aggressive note, here's my set of feedback:
Spell Haste: At the T6 gear level, 1 haste = 1 spell damage. It's not .8 to 1, and that makes a pretty significant impact on gearing.
Gemming for Hit: Never use Veiled Pyrestones ever. You are better off going up to 5/5 shadow focus and trimming points elsewhere. The best places to trim points are Inner Focus first and Focused Mind second. You don't want to do this at lower gearing levels, but remaining hit capped is only an issue in Sunwell gear anyway, when your mana regen can more than compensate for the lost talents. You can in theory live without Meditation but there is no reason to do so. At high levels of gear, focused mind is around 22 m/5 and meditation is 45+ m/5.
Inner Focus: Also, Inner Focus is universally bad unless you are undead. It's less than 20 m/5 even when used with Shadow Word Pain. Undead will still want this talent for devouring plague, but should trim it when they are done with T6 content and need more spell hit.
Shadowguard: This racial is usually bad, but it can be amazing. The three charges combined hit for roughly as much damage as one Shadow Word: Death, without the self damage and without the threat. It also gives Vampiric Touch and Embrace returns. If you are on a fight where this will get activated consistently, it is amazing. On M'uru it functions as a threatless Mind Blast with a 20 second cooldown. A troll shadow priest should absolutely out-damage an undead shadow priest on that fight.
Spell rotations: Once you can sustain it, it's more damage to prioritize Mind Blast over Shadow Word: Pain. This will happen midway through Black Temple progression. Here are the cycles priests should use, from most to least damage. I've included Devouring Plague and Shadowguard in these rotations as well. Ignore if they aren't appropriate. You want to use the highest cycle you can sustain.
Cycle A: Maximum Mana Regen
- Vampiric Touch
- Mind Blast
- Shadow Word: Pain
- Shadow Word: Death
- Shadowguard
- Devouring Plague
- Mind Flay
Cycle B: Near Max Mana Regen
- Vampiric Touch
- Shadow Word: Pain
- Mind Blast
- Shadow Word: Death
- Shadowguard
- Devouring Plague
- Mind Flay
Cycle C: Moderate mana efficiency
- Vampiric Touch
- Shadow Word: Pain
- Shadow Word: Death
- Shadowguard
- Mind Flay
Cycle D: Maximum mana efficiency
- Vampiric Touch
- Shadow Word: Pain
- Shadowguard
- Mind Flay
I also have an even higher DPS cycle that deprioritizes Vampiric Touch on fights where my group doesn't need all the extra mana, like Teron. Regen is lower than the first cycle listed however.
Max Damage Cycle:
- Mind Blast
- Shadow Word: Pain
- Shadow Word: Death
- Shadowguard
- Vampiric Touch
- Devouring Plague
- Mind Flay
You can also downrank Pain and Mind Flay if you are in extremely dire straits. I have rank 3 of each spell on my bar for emergency situations. I do use them but not very often.
Also, you'll note that Vampiric Embrace isn't listed in the rotation. That's because it's normally cast as a rotation filler. For example, you have 1.5 seconds before Mind Blast is up and all other damage cooldowns are waiting as well. If you only have 10 or 15 seconds on Embrace, it's better to refresh the embrace, then Mind Blast, instead of cast a Mind Flay.
Mind Flay Clipping: At higher gear levels, it's really important to clip your mind flays at the 2 second mark for maximum DPS. You should never be casting Mind Flay when there is anything higher damage spell you could be casting instead. There is a custom texture for Quartz that helps with this. I highly recommend it.
Groups: Most of the time, the mages will need you the most. Sometimes healers will need you more, but this is more of a factor of the fights. It's generally a waste to give shadow priests to hunters because we regenerate far more mana than they can use. Judgment of Wisdom plus potions is more than enough for hunters.