I just happened to fight an Elite Possesed with the Illusionist perk at Icefall Caves. It turns out that the copies spawned the expected mobs on death. After a long fight, the floor was completely covered with lots of gold and tons of random junk, too bad I don't have a gold find gear.
To summarize up:
1) Find an Elite Possesed or Grotesque (or whatever variation) with the illusionist perk
2) Equip gold find gear
3) Wait for it to spawn copies
4) Kill the copies, they will spawn additional monsters... that drop loot!
5) Repeat 3 and 4 endlessly
6) Profit!
Notes:
- Be sure not to kill the original mobs, TP to town to let them heal if necessary.
- Use AoE or penetrating projectiles to kill the copies, as they have low hp
- BE AWARE! The copies create wicked chain explosions on death that may choke your CPU/GPU
This is the floor after killing the Elite I told above, a LOT of gold was dropped, but I picked most of it while kiting.