Regarding sockets, here is a summary of tier6:

Absolution Armor - WoWWiki, the Warcraft wiki
Head
meta + blue - 9 +heal bonus
Shoulders
blue + blue - 7 +heal bonus
Chest
blue + blue + yellow - 9 +heal bonus
Hands
blue - 1 mp5 bonus
Legs
yellow - 4 +heal bonus

You will obviously get more out of the item budget if I go for the socket bonuses, but it's down to personal taste. Here are two setups:

1. Going for the bonuses.

Head
12 intellect&restore mana (don't know if that's best, my guess) + 11 +heal/2 mp5 - 9 +heal bonus
Shoulders
11 +heal/2 mp5 + 11 +heal/2 mp5 - 7 +heal bonus
Chest
11 +heal/2 mp5 + 11 +heal/2 mp5 + 11 +heal/4 intel - 9 +heal bonus
Hands
11 +heal/2 mp5 - 1 mp5 bonus
Legs
11 heal/4 intel - 4 +heal bonus
Total stats from sockets: 117 +heal, 13 mp5, 20 intellect + restore mana.

2. Going for +heal.

Head
26 heal & reduced threat + 11 +heal/2mp5 - 9 +heal bonus (going for the setbonus here, since the bonus is 9 +heal, guess everyone will agree ...)
Shoulders
+22 heal + +22 heal- no bonus
Chest
+22 heal + +22 heal + +22 heal - no bonus
Hands
+22 heal - no bonus
Legs
+22 heal - no bonus
Total stats from sockets: 200 +heal, 2 mp5, 0 intellect.

So ... is 83 +heal better than 11 mp5 and 20 intellect?

11 mp5 transform into 825 mana in a 15 min bossfight (1/4 of a Super Mana Potion). 20 intellect becomes rather irrelevant (IMO). If you regen 250 mp5 during the 15 mins (that's not much, actual number will be higher, at least if you are blessed with a shadow priest), your regen alone will be 45 000 mana. The 20 intellect is 330 mana (with BoK).

The value of 83 +heal is difficult to measure, but it scales with both empowered healing, spiritual healing and imp renew - and you get the bonus every time you heal. It will make downranking easier (downranking is still the superior mana conserver) and you will be able to have a higher heal/sec when needed. The latter is in my opinion the most important thing and a good enough argument to go for maxed out +heal (as long as you have stamina to survive any damage thrown at you!). You will find yourself in situations where other healers are dead/running out of rain of fire/etc. and you need to pump out a lot of healing in a short amount of time. You can't achive that "healing burst" with pots (well, [Destruction Potion] ...), but you can always pot mana.

PS! [Superior Wizard Oil] adds healing, personally I prefer +42 healing over 14 mp5, when I'm not using the expensive Brilliant Mana Oil

I'd have to agree. Mental agility gives you 45 mana off the base cost of 450 for CoH. For a spell that when you are casting it you tend to cast it often, it adds up quickly. I would say if you are going to spec CoH you'd be foolish to not spec Holy Reach and Mental Agi.

Shoulders:
[Mantle of the Avatar] vs. [Primal Mooncloth Shoulders]

The Mantle has two blue sockets that give a bonus of +7 healing. Stick in two [Royal Nightseye] to get the socket bonus. So the difference between going from Primal Mooncloth to T5 is: +35 armor, +26 sta, +10 int, +5 spi, +10 heal, -3 mp5.

Robe:
[Vestments of the Avatar] vs. [Primal Mooncloth Robe]

The Vestments have two blue sockets and a yellow socket that give a bonus of +9 healing. The Robe has 1 blue socket and 1 yellow socket with a bonus of +3 int. For the Vestments put in two [Royal Nightseye] and one [Dazzling Talasite]. For the Robe put in two [Royal Nightseye] as I don't feel the +3 int is worth taking the socket bonus. Difference going from Primal Mooncloth to T5: +21 armor, +36 sta, +5 int, +19 spi, -9 heal, -8 mp5.

Belt:
I would pick [Belt of the Long Road] here for comparison vs. [Primal Mooncloth Belt]

Belt of the Long Road has one blue and one yellow socket. Use [Royal Nightseye] and [Dazzling Talasite] to get the socket bonus, though a [Luminous Noble Topaz] would probably be a good choice as well if +healing is the goal. The Primal Mooncloth Belt has the same sockets but the socket bonus is +3 int. Put in two [Royal Nightseye]. Difference going from Primal Mooncloth to Belt of the Long Road (Talasite): +12 armor, +13 sta, +5 int, +22 spi, -10 heal, -8mp5. Difference going from Primal Mooncloth to Belt of the Long Road (Noble Topaz): +12 armor, +13 sta, +5 int, +22 spi, -1 heal, -10mp5.

Overall 3 piece difference:
Belt (Talasite): +68 armor, +75 sta, +20 int, +46 spi, -9 heal, -19 mp5
Belt (Noble Topaz): +68 armor, +75 sta, +20 int, +46 spi, 0 heal, -21 mp5

If the priest has imp DS + SoR + Spiritual Guidance, that 46 spi translates into an additional 17 healing. The spirit also translates into a 4.5 mp5 if the priest is in the FSR 100% of the time, which will double with 2.3 into 9 mp5 at all times. Let's assume the Belt is socketed with the Noble Topaz. How much time do you need to spend outside FSR to gain the 21 mp5?

In 2.2 we will have the 4.5 mp5 from Meditation, so we need to make up 16.5 mp5 from spirit regen.
Our equation is 16.5 mp5 = ((46*1.05) * 2.5 / 4) * x
x = 0.55 or 55% outside FSR.

In 2.3 we will have 9 mp5 from Meditation, so we will need to make up 12 mp5 from spirit regen.
Our equation is 12 mp5 = ((46*1.05) * 2.5 / 4) * x
x = 0.40 or 40% outside FSR.

This does not take into account that the Primal Mooncloth Set gives 5% additional mana regen from spirit while in FSR. Having the Human Spirit or Kings will also affect the calculations.

It's debatable whether to include the two piece T5 bonus as a consideration as that can obtained without having to break the Primal Mooncloth set if you get the Cowl, Breeches or Gloves.

Feedback for calculations or other things that I didn't consider in this comparison would be appreciated.

Remember that if you hit a /stopcast /cast macro in the middle of a spell you will not be able to cancel the next spell by using /stopcasting (due to a UI bug or something like that), the only way to cancel your spell in that case is to move (or jump, which is less prefered). If you're just chaincasting and not canceling you can just use the /stopcast /cast macro again as you're not really trying to cancel anything.

Where do you get 0.65 from?
According to WoWWiki you get 1/4 pr. 2 sec tick pr. spirit, which means you should multiply with 0.625 ((1/4 / 2) * 5) and not 0.65. Unless there is some internal rounding I'm not aware of?

Actually, it heals for 4500-5000 per charge usually (3x~1600).

Some people have used this trinket combined with CoH with very good results. Since no internal cooldown and the proc chance is for each target it hits individually, CoH that hits 5 targets have 5 times better chance to proc than a heal hitting 1 target. Also, CoH mana cost is so low that you will be getting the ooc manaregen.

First of all, 11mana/5s translates into 1980 mana (60 / 5 x 15 x 11) over a 15 minute period of time, not 825. Secondly, you've not taken into account the meta gem at all. One cast per 4.5 seconds is quite conservative and would result in 200 casts (15 x 60 / 4.5). Tat means an average of 4 procs and a mana gain of 1200. Over this 15 minute fight, you would have an additional 3510 mana (1980 + 1200 + 330) to work with compared to your other setup, or asuming the 250 mana/5s figure and 10k mana pool a 6.4% increase in mana. (3510/55000)

Healing, despite what you write, isn't terribly hard to value. Here are some numbers of what an additional 83 healing would provide. Asuming that you go from 2000 to 2083 healing, have 5/5 spiritual healing, 3/5 empowered healing, 3/3 imp renew

Greater Heal Rank 7
(2803 + (0.12 + 3/3.5) * 2000 * 1.0) * 1.1 = 5233
(2803 + (0.12 + 3/3.5) * 2083 * 1.0) * 1.1 = 5322
(5322 - 5233) / 5233 = 1.7%

Greater Heal Rank 1
(1039 + (0.12 + 3/3.5) * 2000 * 0.657) * 1.1 = 2555
(1039 + (0.12 + 3/3.5) * 2083 * 0.657) * 1.1 = 2613
(2613 - 2555) / 2555 = 2.3%

Renew Rank 12
(1110 + 2000 * 1.0) * 1.15 * 1.1 = 3934
(1110 + 2083 * 1.0) * 1.15 * 1.1 = 4039
(4039 - 3934) / 3934 = 2.7%

Flash Heal Rank 9
(1295 + (0.06 + 1.5/3.5) * 2000 * 1.0) * 1.1 = 2499
(1295 + (0.06 + 1.5/3.5) * 2083 * 1.0) * 1.1 = 2544
(2544 - 2499) / 2499 = 1.8%

What is hard to gauge is if a 6.4% increase in mana is better or worse than a 1.7 - 2.7% increase in healing and the reduced threat from the meta gem.

I don't think this should be included, many priests prefer Spell Warding over Inspiration/Holy Specialisation. As most damage you take in raid encounters is magic, it is effectively +10% hp.

Also, Spicy Crawdad should be included in the food buffs section. 30 Stamina and 20 Spirit is a very comparable buff to the ones you have listed, especially when you are lacking hp.

Any priest healer (holy) who doesn't take Inspiration is being selfish.

Inspiration is one of the best, and underrated, talents in our holy tree. It's almost entirely applicable to tank healing only ... but how often do priests end up *not* healing the tank? The only tanks atm who do *not* gain significantly from Inspiration are feral druids, since they are already at their AC cap of 75%.

Spell Warding can be useful, but I'd rather make the tank tougher, and put a PW:S on myself.

Also, re: Lightwell: sorry, I mis-remembered. I'll fix this.

Enchants by slot

Head: [Item not found!]

Shoulders: [Greater Inscription of Faith] or [Greater Inscription of the Oracle]

Back: Unfortunately there are no really excellent options for healers here. Subtlety is a possibility, but many holy priests find that with Blessing of Salvation, some points in Silent Resolve and decent tanks threat is not an issue. 120 armor is, in my opinion, a fairly weak choice for raiding, since any mob that is melee'ing you is likely to one- or two-shot you anyway. I intend to enchant my next cloak with 7 all resistances; 7 is not much, but raid-wide magical damage is common in MH and BT fights and this promises to be more useful than any other cloak enchant.

Chest: Three choices: 6 all stats, 15 spirit, and 6 mp5. I'll refer you to the earlier posts in this thread for information on deciding which of these enchants is more beneficial for your spec, playstyle and current state of gear.

Bracers: 30 healing or 6 mp5 depending on which you feel you need more, I suppose.

Weapon: 81 healing is the obvious choice here; Spellsurge is also a possibility. Refer to this thread for discussion on how and why you might want to use Spellsurge.

Gloves: 35 healing

Legs: [Golden Spellthread]

Boots: Vitality or Boar's Speed. My personal preference is always Boar's Speed since extra movement speed and stamina are extremely useful and harder to find elsewhere than a few more mana regen (consumables!).

Rings (enchanters only): 20 healing

Most of these are no-brainers; if anyone has good reasons why certain enchants are clearly superior to the alternatives I've listed, please comment. I'll admit I haven't theorycrafted the chest enchants in particular so if anyone would like to add some more about them it would be great. I hope this list is useful at least as a base to build on with further information.

Edit: added 6mp5 to bracers, thanks ANSeranov for reminding me.

Nowadays (please refrain in "in the high-end instances, BT and HS) the Priest's role is changed and most healing on maintank is done by Paladins.
You should comment it's not true but think at the encounters like Naj'entus, Teron Gorefiend, Bloodboil, Archimonde, Essence of Souls and so on: the Priest class is more efficient on healing raid than Paladins do. And i'm not considering a CoH specced Priest but an Imp DS specced Priest; if i consider a CoH spec there's simply no reason why a Priest should not healing the Raid.
The fact is every encounter has some critical effect on Raid (a.k.a. no tank&spank in every phase till the phat lewt) and after SSC and TK this effect has amplified.
In SSC/TK Inspiration is worth, not only cause Priest are assigned to MT for their low mana efficiency in fast single heals (FoL vs FL) which are used to refill the raid but for the same reason as the examples stated before: best role for a class. Priests are also used to refill the MT cause Shammy's Ancestral Healing and Priest's Inspiration do the difference in the damage-taken/healing-done math (for how efficient it's the Inspiration there's this old post which old post for whom interested; we should consider this post for the calculation of up-time of Inspiration buff related to the number of Priest in a Raid and for the % of mitigation provided to a T4 equipped tank).
In HS/BT the role is different, as stated previously. Not only cause the equipment for MT is raised in quality but also as Priest you could even put a Renew on main tank and do the raid healing better than a Paladin cause there's a lot of simultaneous environmental damage which can be healed efficiently only by Priest/Shaman, quickier by a Priest (CoH) than a Shaman.

My thoughts.