How come haste is no longer the best stat before mastery?
Of course Mastery is great, "Illuminated healing" is always top 3 healing done, but i'm casting at 2.1sec while raid buffed.
Is that not too long? In term of HPS, how to determinate whether more Illuminated healing, or spellcast will be higher?
Mastery has one big advantage over haste in that it's very mana friendly; you are getting additional "healing" (absorption shield) at no mana cost (where as with haste, more casts = more mana). And absorption/damage prevention has the benefit that it can't "overheal" in the traditional sense; as long as the target is still being dealt damage, the shield will prevent additional damage, meaning less healing is necessary on the target (which means you can follow up with a cheaper heal). This is important since, for the moment, "mana matters."
Cast time is also important however (if you can't get the heal in before your tank dies, the shield is meaningless) so as I said before, get enough haste until you are comfortable. Note that "comfortable" means you are getting the necessary healing to the right targets in a timely manner while not running out of mana. For me, that's at least 25% haste raid buffed, but it might be different for you, so experiment a bit.
As far as a throughput breakpoint for haste vs. mastery, that is entirely situational and hard to quantify from my understanding, and I haven't spent enough time yet in raids to even guess at this.