Overstated relative to cost I can relate too most of the time, but being a myth I can't.
I have heard clear differences made by better cables. In fact my HE6's were my 3rd best phone, did not have the HD800's at the time else they would of been 4th best, until I tried the Toxic Silver Poison and they them suddenly became my favorite.
I have even been blind tested with the stock cable and another manufacturers cable and have always been able to tell the difference and pick out my preferred cable every time.
Overstated that they could very much qualify as myth. The last thing I would recommend anyone spend their $ on. Oh and I speak from first hand experiences. Please feel free to look at my cable inventory.