Well... no matter what your belief is regarding the benefits of custom cables - they do make a very noticeable difference in the sound quality. Though, I don't know that I would consider the improvement proportionate to their cost.
I very much prefer the sonic attributes of silver interconnects and headphone cables - which almost always add clarity, detail and resolution to the sound - at least they have for me. A silver cable was mandatory for the "old version" HD650's to bring out their highs.
Though, I prefer the Cardas copper cable for the "old version" HD600's.
For the "new versions" of these phones, the stock new HD650 cable, seemed to do fine - they didn't benefit that much from an expensive silver cable. Carefully matching the source and amp to them, was far more important.