Being perfectly honest destro's secondaries are very very close to each other so ultimately you're going to see a small difference outside of heavy aoe / high ember generating situations.
Crits your worst stat so you should never reforge into it, I always reforge to get the most mastery / haste possible. Mastery and haste are basically = to each other for general purpose so you don't need to focus one over the other so much as just get the most out of either that you can. There's no exact numbers you're trying to hit with these things.
Also on every boss this tier observer has done noticeably less damage than the fel imp for myself, 2 other locks in my guild, and many random people I come across. I'm not quite sure why this is as on paper observer *should* do more, but when it comes to practice it never does even in patchwerk situations.