These examples are for the average-above average shadow priest.
Shadow in TBC, only used for mana battery, really only competed in T6 with warlocks' Curse of shadows.
Shadow in WoLK, really only competitive at the very end (Pre-Cata release) when dots were effected by haste.
Shadow in Cata, really only competitive when Blizz fixed the expansion long complaints of RNG with the last 4pc bonus.
Shadow in Mists, started competitive but now...
Shadow in WoD... Clarity of Power + Solace & Insanity sounds really good, however, IF it is amazing, then few Spriests will be dotting. Which means blizz will hack either of these two talents, and follow the trend of just nerfing rather than fixing. We always seem to get promises of attention pre-release, but then the nerfs / "normalization" attempts surely come.