We are discussing Dark Piece here right?
Well the power of Dark Pieces depend on how much desires they have at that point. I'm assuming <= to the original's power.
Not once did I notice any sickness in Precia's Dark Pieces thought out the game.
from here:
http://forums.animesuki.com/showpost...postcount=1306
her story mode she beat Chrono,Dark Piece Fate,Shamal or Zafira(depending on player), most likely tie with DP Rynith.