For the sake of simplicity, using lufe's numbers:
During the 21 effective seconds of the buff, he got 11.1k total HPS, and 4.7k effective HPS, just from a use: ability.

It looks like there's a small coefficient boost from either Intellect or SP. Easily the BiS trinket for Holy Priests, possibly even more so than Disc Priests. I'm going to go out of my way to try and obtain it.

I'm guessing this replaces [Althor's Abacus] in healing Priest trinkets?

You should use both. Exceptions apply if you're doing 10-man without Replenishment or some-such. Also, the on-use is super-useful on some fights (Marrowgar, Festergut, Heroic Lich King) and nearly worthless on others (Saurfang).

Ultimately, the trinket is absolutely Best-in-Slot for all of the hardest fights this tier (Heroic Lich King, Heroic Halion, Heroic Putricide, Heroic Sindragosa). One thing to keep in mind, to maximize the use of the trinket, try to spread out your heals to distant parts of the raid to get everyone covered (for example, on Lich King if using the 2 ranged clumps and melee/tank clumb, try to throw heals to all three areas for maximum coverage).

For BiS setups, the combination of the Halion trinket and the Abacus is so clearly best that I put that combination in the guide even before having combat logs.

The reasoning is simple: the alternatives have less spell power, but provide regen. Priests with BiS gear do not have mana problems (exceptions exist, as always). Thus, throughput rules and this combination wins.

Of course, any sane priest will keep very good regen trinkets - just in case.

I have added a very short summary of the latest testing to the trinket section of the guide. Regarding possible scaling, I don't buy the theory of minor stat scaling, though. The increase is just too tiny for a +heal effect at ICC gear level. This looks more like effects from talents and/or +heal effects to me (note the logs show higher ticks of 460 on one of the targets). Given the current state of progression and lack of competing trinkets, it's probably good enough to know the ticks are somewhere between 400 and 500. If someone does more testing outside of raids and with different specs, I will add the results, though.

You're right Hegen! The HoTs are really not affected by Gear.. full epic, or totally naked yields the same hots.. which implies no stat scaling.
I did some more tests with [Glowing Twilight Scale], to figure the scaling out.


Disc Tree
Focused Power: +4% Healing
Grace: Not effective

It's weird that the HoT is not considered a healing received from the priest (Grace innefective), and at the same time it's considered a healing from your spell (Focused Power works). From my understanding these talents should both work or fail. Guess it's a bug.
Hmmm, haven't tested the Holy Talents: Spiritual Healing, Blessed Resilience, and Test of Faith. I wonder if they apply.. If any of you guys happen to check it out, let us know.


Raid Buffs
Improved Devotion Aura/Tree of Life: +6% Healing, calculated together with Focused Power

As I had most of the hots at 443 in Halion, some raid buffs also applies. I suppose it's the ones just listed.


General
ICC Buff: +30% Healing, calculated after talents and buffs



Self-buffed HoTs:
Unspeced: 402 (Base trinket value)
Disc (Focused Power): 418 (+4% ~= 402*1.04)
Disc in ICC (Focused Power + ICC Buff): 544 (+35.3% ~= 402*1.04*1.30)

Raid-buffed HoTs:
Unspeced (Raid Buff): 426 (+6% ~= 402*1.06)
Disc (Focused Power + Raid Buff): 443 (+10% ~= 402*(1+0.04+0.06))
Disc in ICC (Focused Power + Raid Buff + ICC Buff): 576 (+43.2% ~= 402*(1+0.04+0.06)*1.30)

I haven't seen much, if any talk about crit lately on here. Due to circumstances out of my control, after blizzard changed the T10 set bonus, I am stuck in 277 shadow tier for disc, unless I want to lose the stats to drop down to 4 piece 264 holy tier for the bonus, which I do not. When playing a versatile disc role in raids, ie. tank assisting/healing, shield spamming, is there a point we get to where too much crit is overkill? The 'weight' of crit obviously seems dependent upon your raid role, and if you're lacking in other stat areas.

Ever since the change by Blizz to have our shield glyph heals accurately affected by Divine Aegis, DA's healing has subsequently gone up, making crit seem even more appealing.

Ah, you misunderstood Star, or maybe I phrased poorly. I'm not asking which is better. I'm asking about the value of crit at much higher levels. Is there a point to where it become subject to diminishing returns and rather pointless to gain more? Or possible to have too much.

If you're spamming shields and mana isn't an issue, crit is the only secondary stat that makes sense. For anything else, haste is more throughput per point. There isn't a diminishing return necessarily, but depending on your spell makeup some of that critical rating would be better as haste.

edit:

There should be a formula that shows where the benefits change based on your spell usage. For instance, the critical rating conversion is 45.906, while the haste rating conversion is 32.79. So, assuming (falsely) that the throughput increase are the same, haste is already 40% better per rating.

However, for direct heals, Haste gives a flat 1% increase per percentage. Critical gives less, since a critical heal is 1.5 times larger than a normal heal instead of twice as large. I believe that Divine Aegis makes this 1.9. For shielding, haste gives 0 value, and critical gives an increase that is 1.9*20% of your shield value (Power Word: Shield glyph heal).

Perhaps one of the more advanced statistical mathematicians here can confirm the formula that shows at what ratio of direct heals to shields does haste come out as the throughput winner for Discipline priests. This is particularly relevant for Halion if you are the 'upstairs' healer, and a large portion of your heals are direct heals on the tank.

As all of you are aware, the 4.0.1 patch introducing heavily changed mechanics will hit servers any day now. The applicability of the current Priest Healing Compendium thus comes to an end and so does this thread.

In my final revision to the guide, I have appended a new section that contains my notes regarding which guide section needs some work, needs to be rewritten or might still be usable (I was actually surprised there are such sections). If a new guide author wishes to discuss that list, please use PM - expect this thread to be locked pretty soon.