So I made myself some stat weights, estimated set bonus values and plugged it into a spreadsheet.
The first thing that stood out to me was that the 4set is significantly weaker than the 2set. This led to an interesting revelation that I don't believe there's a significant advantage to taking the 4set bonus if optimal offpieces are available.
Talking full BiS here (including Archimonde);
Pious Cowl 725 (tier, haste+multistrike)
Locket of Unholy Reconstitution 730 (spirit+multistrike)
Pious Mantle 725 (haste+multistrike) Mantle of the Eredar Lord 735 (haste + multistrike)
Cloak of Hideous Unity 730 (haste+multistrike)*
Raiment of Divine Clarity 725 (crit+multistrike) ** Pious Raiment 725 (crit + haste)
Contemptuous Wristguards 730 (crit+mastery)***
Satin Gloves of Injustice 730 (crit+multistrike)
Demonbuckle Sash of Argus 735 (mastery+multistrike)
Leggings of the Iron Summoner 730 (crit+haste)
Bloody Dagger-Heeled Pumps 730 (crit+versatiliy)
Ring of Foul Temptation 725 multistrike+spirit)
Demonic Phylactery 725
Iron Reaver Piston 720 (int, crit, haste)****
Edict of Argus 735 (haste+mastery)*****
*No spirit here but it's also 5ilvls higher than the spirit cloaks, I think dropping some spirit will be quite fine especially with trinket options
**This stood out as odd to me since it's not an ilvl increase over tier, but the itemization seemed to make this a more valuable offpiece than upgrading the ilvl on shoulders.
*** worth noting this is VERY close in value to Powder-Singed Bracers. Likely to swap places depending on specific stat weights. The fact that Powder-Singed Bracers have favorable itemization and are available from the first boss in the instance might make these the better choice - especially if more kills give more chance at wf/socket etc.
**** DPS trinket so good luck getting it. Flickering felspark would be my 2nd pick. A few reasons behind these choices but probably deserves it's own post. In short; leech trinket doesn't stack from overhealing. throughput procs suck. these static trinkets seem overbudget (compare 690 piston to 691 quiescent runestone)
***** yeah, good luck with this one. Very close (and potentially better depending on specific stat weights) is Gavel of the Eredar + Gibbering Madness combo. I'll be aiming for the mace+OH due to less competition and only being minutely behind. 3rd best option is the Voidcore Greatstaff. Which should be your preference for Archimonde progress, it's not that far behind Edict of Argus either, if this came up with some extras I'd certainly take it and be happy.
This results in;
4954 int
2545 spell power
1772 spirit
1101 multistrike
2727 haste
585 mastery
1678 crit (:<)
148 versatility
average ilvl 728.44 (with 715 quest ring)
Lots of haste+multistrike items, not taking a spirit cloak really stands out as the obvious choice here since the other spirit slots are generally well itemized anyway with multistrike, as it stands the Demonic Phylactery is going to be pulling A LOT of weight in terms of mana, it's by far and away #1 on the priority list if you're going in with BrF mythic gear. The piston / felspark choice should be interesting, will need to see what mana is like during progress but it's a flexible spot and the numbers are putting them both quite equal, so if you had access to both then this would be my slot to change as needed.
yeah, no 4 piece. Since my maths is telling me they're 'close' I wanted to entertain the idea of not taking 4set. To further that, I believe in a realistic raiding environment, and looking at other class setbonuses, I'm FAR more likely to be able to pick up some offpieces with WF/Socket/Leech etc than I'll be able to get with tier. Obviously that's unfair to include in a BiS list since BiS is BEST in slot. Even so I think for planning purposes it's worth keeping in mind.
The list also only contains 2 items from Archimonde. One being a weapon which will be in high demand by all casters, the other being a belt. Again the belt is likely to be in high demand by all casters but we may be the luckiest since our '2nd best' option isn't that bad at all.
Kind of sad that there are some perfectly itemized drops that become irrelevant as you progress through the later bosses simply due to ilvl although on the bright side it does mean that the gap between being geared in the first tier of the instance and the end bosses will be much smaller for us than some other specs.
And just for reference, swapping out those higher ilvl pieces to better itemized 'worse' pieces, looks like;
Pious Cowl 725 (haste + multistrike)
Locket of Unholy Reconstitution 730 (spirit + multistrike)
Pious Mantle 725 (haste + multistrike) mantle of the Eredar Lord 735 (haste + multistrike)
Cloak of Hideous Unity 730 (haste + multistrike)
Raiment of Divine Clarity 725 (crit + multristrike) Pious Raiment 725 (crit + haste)
Powder-Singed Bracers 720 (haste + multistrike)
Satin Gloves of Injustice 730 (crit + multistrike)
Sludge-Soaked Waistband 720 (haste + multistrike)
Leggings of the Iron Summoner 730 (crit + haste)
Pedal-Pushing Sandals 720 (haste + multistrike)
Ring of Foul Temptation 725 (spirit + multistrike)
Legendary 715 (spirit + haste)
Demonic Phylactery 725
Iron Reaver Piston 720
Gavel of the Eredar 735 (haste + multistrike)
Gibbering Madness 725 (crit + multistrike)
4875 int
2544 spell power
1772 spirit
1819 multistrike
2952 haste
0 mastery
1310 crit
0 versatility
average ilvl 725.63 (with 715 quest ring)
Looking at those stats they look much friendlier for holy priests
That said the difference between the sets is only 28PP (half a gem) in favour of the higher ilvl with 25PP of that from the belt alone. Swapping out the weapon to voidcore with those lower ilvls items is still 'only' ~115PP behind full BiS. Considering the ilvl difference between the sets I'd say that's pretty good. Both in terms of flexibility and availability.
That 2.8ilvl increase will net you an impressive (/sarcasm) 0.24% increase in PP.
Generally happy holy priest with this itemization
Finally for the items I'll actually be aiming for for Archimonde progress, this list is a combination of the 'better itemization, lower ilvl' due to availability (hopefully) and dropping off earlier bosses hopefully resulting in more of it with bonus stats;
Countenance of the Revenant 730 (haste + multistrike)
Locket of Unholy Reconstitution 730 (spirit + multistrike)
Pious Mantle 725 (haste + multistrike)
Cloak of Hideous Unity 730 (haste + multistrike)
Pious Raiment 725 (crit + haste)
Powder-Singed Bracers 720 (haste + multistrike)
Satin Gloves of Injustice 730 (crit + multistrike)
Sludge-Soaked Waistband 720 (haste + multistrike)
Leggings of the Iron Summoner 730 (crit + haste)
Pedal-Pushing Sandals 720 (haste + multistrike)
Ring of Foul Temptation 725 (spirit + multistrike)
Legendary 715 (spirit + haste)
Demonic Phylactery 725
Iron Reaver Piston 720
Voidcore Greatstaff 730 (haste + multistrike)
4871 int
2428 spell power
1772 spirit
2155 multistrike
2704 haste
0 mastery
1217 crit
0 versatility
average ilvl 725.31 (with 715 quest ring)
EDIT; after a little further tinkering I'm not convinced it will be worth dropping 4set until mythic Archimonde loot becomes available, specifically to gain the shoulders as an offpiece. The gloves would be the go-to offpiece being worth almost double the upgrade of any other offpiece slot. This also largely depends on the increase offered by the 4set. My very basic testing and maths put the 4set at around 2% increase in healing, I'm very very open to more accurate numbers here though
PS;
fully expect set bonus changes and probably trinket changes before live. I'm just speculating a BiS list for funsies
EDIT; updated thanks to Mylliors catch of my typo