Checkout this study done on two different cables.
I did not check everything but several of the results are exactly what I would have expected based on physics. The zip cord is not as good and one of the wel known disadvantages of thicker wires versus many small wires will indeed be a slight roll off above 15 KHz (skin effect). This appears on the plots. Phase difference will also be expected to be higher due to the wire separation in the zip cord (the wires are close together in the Clarity 7) - again the results look intuitively correct.
Bear in mind that a 9 KHz square wave has frequencies well up into the 100 KHz range (as it is square) - so I am not sure of the relevance of this plot - or the one with resonance.
All in all I woudl support the claims that the Clarity 7 is better than the zipcord - although this is mainly true from 15 Khz to 400 KHz. As for me I can't hear above 15.5 KHz so I am not sure if I shall be switching from my monster cables ( basically zip cord or not different from zip cord ).