Hey, let me offer you some insight on those spells and some consideration.
I am one of the lock players who help testing, theorycrafting and implementation of SimC warlock, i did quite some of the testing around those spells and i believe i have some idea of the behavior of those spells during the PTR.
About Demonfire Infusion
I will be talking about this as %chances but truth is that the system is less RNG and follow a accumulator system with variance, this means that everytime you cast Incinerate for exemple it gives an value who averages 0.25, once it adds to 1 it represents a proc.
So on average you will see it happening every 4 incinerates but with some variance (its possible to see a cast happen after 2 casts of Incinerate, but also see it only after 5~6, both are rare).
Talking as % procs is just simpler given the tooltip description and the variance in the system.
1º In single target it will offer less Demonfire Bolts per second than Channel Demonfire, this happens because the 25% proc-chance on Incinerate + 4% proc-chance on immolate can’t catch up to Channel Demonfire 15 bolts every 25 seconds.
2º Demonfire Infusion on the other hand, offers better AOE growth due to two reasons.
A - It synergizes with Fire and Brimstone in the Incinerate, this growth is not linear, it have diminishin return behind it following a cubic formula, effectivelly at 4 targets you have 60% chance to proc 1 Demonfire Bolt, effectivelly you will see 1 proc every 2 casts instead of an average of 1 proc every 4 casts.
B - It allows it to scale with nº of targets on Immolate, as each targets is an additional 4%(again accumulator not a real proc-chance) ticking.
As such, as target count increases, Demonfire Infusion eventually catches up to and may even surpass Channel Demonfire at High-target counts.
About Dimension Ripper
This talent function in a similar way as B) from Demonfire Infusion, for each target you have with your damage over time effect the faster/more it will happen.
Currently (at least last week from PTR it was) it’s bugged with a value of “2.25%” in its accumulator instead of the 5% so i can’t recomend it at those values in any sensible way.
IF, they come to fix this issue (or have fixed with the patch release) it will be lower single target option than Dimensional Rift but with potential to surpass it in aoe scenarios due to the target-scalling of additional damage-over time effects.
This have been fixed, it only happened on Hellcaller, if it had made to live you would play with it as it would far surpass CDF in AOE and slightly better in single target.
I personally wouldn’t say it’s a completely dead talent yet, it may see use in m+ due to the aspects i mentioned above (target scalling), but it does come at a bit of an single target sacrifice.
the loss isn’t as high as it looks because as a passive talent you don’t need to spend GCDs(button presses) to gain its effect.