I'll be adding this into the FAQ, but I'll post here:
Theres two parts to this. One is a council fight. Agony has a dropoff on shard generation that can me modeled like this (Props to Rez for making this kick ass spreadsheet):
Basically going from 2 -> 3 targets only provides a 20% increase in shards, and from 3 -> 4 targets is only about a 15% increase. What this translates to, is for sustained 2 target fights, you get "A similar" number of shards going ether effigy OR conduit, and any more targets conduit will win out on shard generation. As far as damage, its not 100% confirmed. On single target, effigy is going to be ~10-15% of your damage pending on talents / encounter. I would GUESS that 3 target is where you will see enough shard increase from conduit to make up for the damage lost on effigy + the GCD loss from maintaining a minimum of agony on 4 targets instead of 3.
On add cleave fights, which theres 2 of in nightmare, you will gain more out of conduit if your raid needs you REALLY focused on adds, and less if you are just throwing stray UA's out on adds that are closing in on death (Remember you get the shards back if they die with UA still on them) and focused more on boss damage. Also: Any time you have to replant effigy more than once or twice in a fight, you will want to run Conduit. Ilganoth mythic you will PROBABLY want to run conduit becuase theres no real places to put an effigy up (this is all pending on PS not lining up to be useful on this fight).