The demonology Tier sets are on the low end in terms of single target output gain, but they scale well into AoE scenarios.
The 2 set increases Felstorm and demonbolt damage by 20% and is pretty nice for burst cleave paired with Demonic Strength. Demonbolt focused builds will also benefit greatly from this bonus with all the demonbolt amplification talents available.
The 4-set bonus reads, “Demonbolt has a chance to make your Hand of Gul’dan Instant and do 150% more damage”. This is not as crazy as it sounds. Probably around 2% Single Target and 3.5-4% AoE. The instant cast part of the bonus offers some mobility but I don't see it being super impactful.
For general trinket use, we are still waiting on sims, but a good bet so far for a trinket duo is:
The Irredius Fragment trinket is unavailable from Mythic+ in season 1, but for Pit Lord builds, it could end up being BiS even at Mythic 0 due to its insanely high initial int value on a 3-minute cooldown.