I've been doing some experimenting with simulationcraft on Wabe (my balance druid) to pick gear sets, both with the default Patchwerk fight style and the "HecticAddCleave" style which simulates a fight like Horridon with periodic add spawns and movement requirements. Some notes:
* Lady and the Child (which boosts moonfire damage and causes it to hit an extra target) was broken in simulationcraft until recently. Now that it can be modeled correctly (I hope), it's a pretty dramatic boost to damage in the Horridon-style fight, coming in at 25% more DPS over normal T19 shoulders. For a Patchwerk-style fight it's predictably underwhelming, at a 4% boost, most of that from raw item level.
* The Emerald Dreamcatcher (which causes starsurge casts to lower the cost of starsurge for 3 seconds) simulates at a 9% boost damage over a normal T19 helm for Patchwerk, and 13% for Horridon. That second result is confusing, and I don't really know what accounts for it.
* Kil'Jaeden's Burning Wish (which does on-use AOE damage near an enemy target with a 75s cooldown) is a strong trinket at 14% vs. Patchwerk and 20% vs. Horridon over no trinket. Other trinkets I have are in the 4-8% range. I'm using a Star Gate in my multi-target set (8% vs. Horridon) and a gemmed Unstable Arcanocrystal in my single-target set (7% vs. either Patchwerk or Horridon).
* The T19 set bonuses are strong against Patchwerk, at around 4% each, but are weak against Horridon at only about 1% each. I'm still using 4pc T19 in my multi-target set, but if I had really strong offset pieces I'd consider abandoning the 4pc bonus.
* I don't see the same stat weights as the 7.1.5 guides see. Against Horridon in my multi-target set I see haste being very strong (~39) and the other stats, including int, all being about the same (~17), even though I have a lot of haste. Against Patchwerk in my single-target set I see int at ~14, mastery at ~13, and the other stats all very close together at ~11. To me this reinforces my belief that stat weights are very dependent on your current gear and on the parameters of the simulation tool.
On a side note, Lady and the Child also works for feral with Lunar Inspiration and for guardian. For guardian with Galactic Guardian, moonfire after a proc will hit two targets and generate 16 rage instead of 8. Lady and the Child does great damage on trash, but you have to be very careful when using it, as it will prefer an out-of-combat target within 20 yards with no moonfire debuff over an in-combat target which already has moonfire.