Shadow priests rely alot on gear, specially trinkets and your weapon (main Spell Power source), most of the top Shadow Priests right now have mixed feelings about trinkets, and which exact setups to run for. But mostly, we all agree that FDCL (From Darkness Comes Light) and ToF (Twist of Fate) are generally the way to go when it comes to talents, and that UVLS (Unerring Vision of Lei Shen) is probably your best trinket, and that you should focus on running for Mastery right after you reach the 14846 haste breakpoint (15k), since this way you will get the arguably best stable dps for a shadow priest.
About the DPS, Shadow benefits from raid buffs like no other class, as example, my ST (Single Target) dps is around 230-250k at the Raid Dummy (100mill runs), but in a raid, versus a single target boss with no adds my dps will usually be around 380-450k, more with luck, less with worser luck. (This is without taking in mind buffs like Twist of Fate, that you can force to proc outside of their normal behaviour, by for example healing low HP players while dpsing to benefit the most out of it).
My current setup is the following, i'm running 15k haste > Int > Mastery 50% > Crit 30% > 18.2k haste > Crit+ = Mastery+. Currently at 571ilvl with ToT (t15) 2p and SoO (t16) 2p and UVLS + BBY (Black Blood of Y'shaarj). This is giving me around 380-450k single target dps on bosses where i cannot benefit from ToF over 30% uptime, and on bosses that there are multiple adds, cleave, or that i can multidot, my dps ranges from 600k-1.1m depending on the situation. My main DPS source is based on Mind Spike procs, and ToF uptime, im always focus on trying to have as much ToF uptime as i can, hotting friendly targets or doting low hp enemies, my average uptime on said talent is 45-55% at Siege of Orgrimmar, some bosses much higher uptime.
Keep in mind the following, your main source of damage are Trinket uptimes, and Buff uptimes like ToF, you should always prio your ways to do so, let it be healing others, or casting soft damage aoe skills (like Mind Sear) on 2+ targets just to forcefully proc your trinkets.
As Megicha mentioned concerning Shamans (Toxic Totem) trinket and BBY trinket, if you were to donate, dont get BBY right away, i personally have a much greater uptime on Toxic Totem, than on BBY, and by much, i mean 40%+ uptime ST dps on Totem, against 15-30% uptime on BBY, the only reason of why i keep using BBY is because it gives me haste, giving me the chance to redirect offstats to mastery + crit, and been able to use mastery + crit items over haste ones, granting me a higher flat dps increase with gear.
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