We've done very little in Firelands as 25 man, but we're making steady inroads in 2 ten man teams and I'm starting to get some gear together which made me rethink stats and all that jazz. If you've been looking all over the web for simple numbers, trust me you're not alone.
I can give you this much; this is my opinion based on what I've found on teh interwebs where clever people have number crunched for me.
Intellect - spirit - haste to 10.08% (and no more*) - mastery -=- crit.
- Intellect. If I have to tell you why please go roll a warrior and tank. Thanks.
- Spirit. Meditation (a passive ability that will allow 50 percent of the mana you'd recover outside of combat to occur in combat.) is still the main reason why we need spirit on gear. You want that mana.
- Haste. Apparently to get your PoH down to 2.1 seconds casting time (and an extra tick on renew) you need 1290 haste or 10.08% haste rating. That's an easy enough threshold to reach and to get it to 2 seconds is an insanely higher number that would drop other stats too far IMO. Borrowed time will allow you to get higher haste but it's not easy to always have it active now we're not bubble botting.
- Mastery beefs up our absorbs. Even if you're only shielding the tank once every 12 seconds for rapture to proc, then it's worth it.
- Critical strike. OK, so the crit buff means that divine ageis is proccing off crits, but PoH will always put DA on a target anyway, crit or no crit. If you have enough of everything else (spirit on nearly every piece possible) for instance, then don't reforge out of crit. If you get a piece with haste that'll take you way over 10.08% consider reforging to crit if you can't put spirit or mastery on it, or juggling the +haste on other pieces that you can change - +mastery enchant on gloves for example.
Right now I'm still reforging crit to spirit, when I can't reforge Crit to spirit, I'll take the haste instead and use the extra haste - spirit or mastery on another piece. It's all about robbing Peter to pay Paul and getting round the limitations of what you can change on each piece in each slot.
If you're doing a lot of tank healing you'll value crit more, if you have to role switch or are mainly raid healing mastery will be the way to go. You might get away with less haste and more crit as a tank healer too, but I've said not to undervalue haste many a time and been proven right when my haste dropped below the 10.08% by having deaths while my heal was casting in big damage phases. TL:DR
At higher gear levels haste crit and mastery are coming out about even. Don't stack any to the detriment of others. If a piece has more intellect than you have with no spirit, it is an upgrade BUT only take it if a DPS doesn't want it as it's not optimal for you. Reforge haste (if you're over 10.08%) crit or mastery to spirit. I favour reforging out of crit still.
*With 2/3 Darkness & 5% from raid buffs: