Diamonds in weapon don't do anything. Conduit does not scale with any player damage stats (like elite damage, elemental, DPS, and so on). For more on that read the pylon guide.
I assume you're talking about the Twister meta, right? Conduit is good to kill what you are not good at - and that is single target high HP mobs. So essentially, like leftover yellow/blues. What you want to make sure is that you kill as much trash as possible, and if you have the choice to pull in an additional pack check if it's Illusionist. Illusionist is bad for Conduit.
Conduit has a few little beams (I thought around 5, Michael recently said it's 3, not sure) that alternate between enemies. They're not hitting the same enemy constantly, but basically go back and forth, so optimally you'd have 10-15 mobs or so that are all high HP and no trash. The Conduit beams will then alternate between those and bring them down. Any trash or iillusions will "soak up beams" and reduce the efficacy of your Conduit.
"Sometimes we kill 5-6 elites" sounds like a huge overstatement; the very top clear videos show people killing 4, sometimes 5 elites. 6 elite packs are 20% from the progress globes alone, probably around another 20% depending on mob types from the mobs itself, so that's a 40% Conduit. That usually only happens in Torment 10 rifts, but not when pushing. ZE had a Conduit and 4 packs in their first 117 attempt yesterday and probably made around 25-30% - and that was pretty good. (Failed the rift because of level 2 Sewers map and a few other things).
Anyways: No, Conduit is not nerfed. To kill 4 or more elite packs, you need to make sure they are rather isolated (no trash, only few minions) and not Illusionist. As long as there are a ton of mobs on screen, especially when densely packed, the wizard's Twister+area damage will deal more damage than Conduit.