Sometimes, yes. I can't listen to Metal with the stock cable, and some very sibilant recordings (Diana Krall, Eva Cassidy Live, Kathleen Edwards) will sound more sibilant with the stock cable than the Warren Audio double helix or Locus-Design Hyperion Cable. But, with the right amp and music, the HD800 is very good with the stock cable.
Agreed. With the right source and amp, the stock cable is not sibilant at all (to me anyways). It's just not a forgiving cable so bad recordings sound bad, whereas a copper replacement will make pretty much anything sound good because of its smoother, warmer and more open sound.