FYI: You can Cap GCD on 101 Disc Priest with 50% Haste. At that point you have several options depending on your play style. More Crit is never a bad idea though I wouldn't push it much past 45% if you regularly do group content. Mastery is great if you like to go balls deep in DPS without too much care for actual Healing because of how much "Leech" Atonement provides your friendly targets (that you've healed/Bubbled in the past <15 sec). Lastly Versa is also a solid choice, especially for WPvP. Here's my basic recommendation you can test for yourself to see how well it works for you.
- WPvP / Solo PvE: Haste (to 50%) > Versa spam >= w/e Crit your Gear gives you >= Mastery; with as much Leech** as you can get, and possibly Speed on Helm.
- PvE Group: Haste (to 50%) > Mastery spam >= Crit no more than ~45% >= w/e Versa your Gear gives you; with Speed on Helm*** and Leech the rest.
* (2) Set
T18 HFC Mythic is recommended for Bracers/Boots for Penance damage. Considering that HFC Mythic can proc as high as 775 (785 upgraded 2/2) as well as proc a Socket and Tertiary it might actually be beneficial to go (4) Set since Cloth doesn't have Legion Epics for Chest/Legs. You'd lose 15 iLvL each pc but the gain of (2) Sockets and 2 more pcs with Leech may outweigh those 15 iLvL. This is ofc assuming you can even get a 775 proc + Socket + Leech even after years of weekly farming.The
Class Trinket just reduces damage taken up to ~11% for max iLvL for 10s after most Abilities used. Probably quite good for WPvP.
** It should be noted that Disc Priest can achieve well over 100% Leech with 100% uptime and upwards of ~150% Leech for 20 seconds every 2 minutes thanks to
Eyasu's Mulligan. Even at 835, that Trinket's on use is ~50% Secondary Stat which equates to a direct +50% Atonement Healing.
*** The reason for only having Speed on the Helm is due to Blizz's stealth nerf to Movement Speed. At some point since 7.0 (Legion) Blizz implemented DR on Movement Speed at 110%. ONE item with Speed will not be effected by DR but a 2nd will. The Helm has the highest Tertiary Stat allocation (along with Chest, that Cloth doesn't have). That means at 895, a Helm with Speed should give you about 16% Movement Speed with a single item. The WoD 2% Gift Enchant stacks as well as the 10% from
Minor Speed, but you'd have to go get an iLvL 600 pair of
Boots from Skyreach to Enchant with Minor Speed, losing much of the benefit of higher iLvL Gear.
One thing to note, if you Enchant those iLvL 600 Boots THEN upgrade them to 610 with Valor, the Minor Speed Enchant still works. If you also happen to get the Speed proc on them that'll give you a total of 12% Movement Speed (10-11% with DR). And yes, Minor Speed is actually 10%, regardless of what the TT says.