Scale Factors (5 seconds of movement every 30)
I think you're basing your opinion that mastery is only slightly better than crit in bis gear on a turrent fight on old versions of simcraft. Get the new ones instead and you'll see that there is a significant difference (it's even more in favor of mastery for me).
Also, the more I think about it I can safely say that I rarely run for as long as 5 seconds every 30 seconds, it's only on a handful of fights and on some of them I'm only running because I am out of range. 2 or 2.5 seconds (sometimes less/more depending on whether you get targeted by things or not) feels more accurate for most fights.
Unlimited mana, helter skelter fight, 5 sec movement/30 sec: 0.5469 haste; 0.4026 crit; 0.4012 mastery
Limited mana, helter skelter fight, 5 sec movement/30 sec: 0.2969 haste; 0.3286 crit; 0.2919 mastery
Unlimited mana, Patchwerk fight, 5 sec movement/30 sec: 0.5013 haste; 0.4064 crit; 0.4394 mastery
Limited mana, Patchwerk fight, 5 sec movement/30 sec: 0.3487 haste; 0.3924 crit; 0.3639 mastery
Change: NO devouring plague spamming unless it has to be refreshed on the two following sims.
Limited mana, Patchwerk fight, 5 sec movement/30 sec: 0.5041 haste; 0.4116 crit; 0.4288 mastery
Limited mana, helter skelter fight, 5 sec movement/30 sec: 0.5258 haste; 0.4026 crit; 0.4216 mastery
What I understand from this:
Haste is superior on both helter skelter/patchwerk fights where either mana is unlimited or devouring plague burst spamming isnt allowed, basically making it my most wanted stat.
Crit is superior to mastery on both Patchwerk and Helter Skelter fights with limited mana and devouring plague spamming allowed. Equal to mastery on fights helter skelter fights with unlimited mana where devouring plague is spam-able.
Mastery is superior to crit on fights that includes unlimited mana, less movement/rng, where devouring plague isnt spam-able.
As a shadow priest you will most likely have unlimited mana, wont be able to spam devouring plague because of SP modifiers, however most fights will include RNG/movement. Mastery will also scale better on multidotting fights where DoTs will do an overall higher damage and multidot-SW:Ps will make up for the loss of shadow orb generation because of movement/RNG.
Overall I'd say mastery is superior to crit, thanks for correcting me.
PS: I don't have the (4) set bonus which makes crit scale better. The new T12 set bonus will most likely make crit scale better too and make it preferable (assumption).