DP3 due to occasionally having 2-300ms just makes playing at GCD unbearable at times.
I did a look around on wowprogress at the first few pages of best geared Shadow Priests. The results vary so much!
GCD then stack Crit, GCD then stack Mastery, GCD then balance Crit/Mastery, DP3 then stack crit, DP3 then stack Mastery, DP3 then balance Crit/Mastery.
Having tried all of them, I think the DPS difference overall is minimal enough that you should go for what you "feel" to be better for YOU.
I'm currently at 42.11% with 9,397 crit rating and 9407 mastery rating. I think it's a nice balance especially when I switch between FD;CL and SW:I depending on fight. Especially when you see those 1m procs! (Not that this has anything to do with haste, only the fact that you get a bit more out of Crit/Mastery if you go for DP3)