I would like to meet the person that said: "you must have 1400 MD and full T4 before you can sacrifice KotU." It is absolute ********. It depends fully on your combo rotation and the encounter you are doing. In my opinion you waste too many points by taking KotU and not progressing further in a Possession tree already quite deep. It gimps both trees. Most classes has interesting stuff on opposite tree when you want to make a one side heavy build, and it is not always worth it. Need to remember coefficients in this calculation. 100 MD does not add 100 damage to your spells. They are divided by a specific number. I had the list of coefficients earlier, but seem to have lost it.
If you don't have ToS (Curse of Set) or Demo (Chaotic Blast) in your group, you need to be using your debuffs. Saving 1 second on the Inferno Curse to make it instant, and give you -15% elemental invulnerabilities for 15 seconds on the mob is a nice dps boost. Molten Steel Slash gives -10% elemental invulnerabilities for 30 seconds. So just by doing proper debuffing you remove 25% of the mob's invulnerabilities. And making one of them instant rather than 1 second actually "give you" damage in addition to doing more damage after they are applied.