Well I’ll tell you a little of what I have found ( keeping in mind I may be wrong). I have spent a fairly large amount of time at the Practice dummy to figure this stuff out; so , I kind of want people to respect that. But, the big secret seems to be Demon uptime. As far as I can tell all demons you spawn with nether portal has a 15 second uptime or really close to that. Summon dreadstalkers only has a 12 second uptime. Summon vilefiend and Grimoire:felguard has a 15 second uptime. Nether portal portal is up for 15 seconds and it takes 2 seconds without haste to summon your demonic tyrant. So, the problem I was having with my rotation is I would wait until nether portal would fade to cast demonic tyrant. The problem is 15 seconds plus 2 seconds is 17 seconds. So, my vilefiend had despawned and a few of the demons summoned by nether portal had despawned. So, you have to find the sweet spot with your haste level and make sure you tyrant is out before any of them despawn. Summon dreadstalkers should be toward the end of the list. When I did the Icy veins rotation exactly as written all my Major demons would get the extention. Then I had a little bit of time to spam a couple hand of gul’dans.
This has led me to a number of assumptions, which I have not yet concluded definitively. But, the main one is Nether portal is only a damage increase if you pair it with Demonic Tyrant. When I was screwing up the rotation trying to maximize my nether portal window, the dmg was always lower then sacrificed souls. Because, I did not get an extention on all of my major demons. So, because of that I can conclude that if I am not extending my demons summoned by nether portal it is not a dmg gain over sacrificed souls.
So, that means IF you want to be a super pro Mythic raider and not just a LFR pugger you are going to have to analyze the encounter and decided when you can use abilites and when you can not.
Which brings me to another point. In order to maximize your nether portal window. You have to spend 15 seconds summoning demons. But, I did not hit my peak DPS numbers until about 5 seconds after nether portal had faded and then the burst window carries over for about 20 seconds after that. So, that means you need about a 40 to 45 second window to maximize the dmg from nether portal. If the encounter allows that and you don’t need to save burst for some reason. You are golden. But, do keep in mind that I have not conclusively determined this.
In situations when you can not just faceroll the boss for 40 seconds sacrificed souls still might be better. But, I have found that in the heat of combat it is difficult to manage imp uptime. Which is how sacrificed souls works. If you can not effectively stack imps then your dmg is going to be better with nether portal no matter how you roll the dice. Demonic consumption does have a place. But, when I use it my sustained damage is not good. But, it gives a very powerful burst every 1 min 30 secs. The burst window is faster and higher then nether portal burst window. But, does not last as long and does not provide the same over all dmg. But, if you were in an encounter when you had to burst really hard every 1 min 30 seconds and there might even be a bit of down time between burst windows. demonic consumption might be a good choice.
But, other than that each build has its own hang ups. Sacrificed souls relies on your ability to stack imps. Weak auras works well for that. Demonic Consumption is similar but works a little different where you are looking at IMP energy instead of the total number of imps. Which is more difficult to calculate with weak auras that I know of. Nether Portal on the other hand seems to rely on Pairing it with demonic tyrant and making sure as many demons get the extension as possible and all of you primary demons get the extension.
In order to get this correct you will have to spend some time on a practice dummy. Because your haste levels will differ from other people.
Have a Merry Christmas