I found this site the other day and thought it was relevant to this thread: http://www.roger-russell.com/wire/wire.htm
Despite the mid-90s vintage website, the guy knows what he is talking about. He was the Director of Acoustic Research at McIntosh Laboratory, Inc. and the originator of McIntosh Loudspeakers. He is retired and now designs and sells his own high end speakers.
What I take from it is that there may be other attributes (like construction quality, durability, ease of connection) that justify buying a more expensive cable, but as long as it is rated to do the job, you are wasting money paying for better sound quality because there is no difference in sound quality (again, provided you are using a wire/cable of sufficient diameter).