There are no reproducible peer-reviewed results that indicate that cable material makes any difference whatsoever. That should tell you what you need to know.
Rational reasons to buy a replacement cable include comfort, getting a customized length, and aesthetics. I bought one for these reasons.
I politely disaggree. I bought the Wywires Red and immediately noticed an opening in the treble and stage of my Xs. On a sidenote, I noticed a great upswing in the sound of my 846s from the Ted Alan silver lids. I'm in the pro-aftermarket cable upgrade camp. I'm a piano tuner and take great care of my hearing. IMHO