I have to agree with what so many others have said, that if you have a good setup the upgrade cable will definitely be an improvement. It's so funny how different cables will work with different setups.
I have the Cardas cable for my 650's. From what I've read, most would concur that the Cardas is a very good cable and a huge upgrade over the stock cable. With that said, most people prefer the Zu, SD, APS, or Equinox over the Cardas. In my setup, the Cardas is perfect. I've got a slightly warm CD player in the C542 connected to my Rapter(a more forward sounding tube amp) with the Signal Cable silver reference rca's which definitely brighten up the sound. The Cardas does a great job of taking the veil off, pushing the mid-range more out in front, but still keeping the 650's great bass.
It definitely has more detail, but is not overly bright, and maintains that nice warm bottom that Senn's have. I think a silver, or a silver plated 650 cable would be too bright in my system. Maybe someday I'll fork out the cash and do a test, but for now I can only hypothesize.