OK, well, I finally bought an Audioquest Carbon HDMI cable to put this to the test myself, i.e., whether or not there would be a difference in picture or sound quality versus a basic high speed cable.
So, my first test was to just replace the basic HDMI cable running directly from my cable box to my TV with the Carbon. I switched them back and forth maybe 15 times. I just cannot tell any difference. Now, one caveat is that the cable box will not output any higher than a 1080i resolution signal, so perhaps that could explain it.
My next test was to compare audio quality. I replaced the basic high speed HDMI cable that goes from my Oppo BDP-83 universal player to my Denon Receiver with the Carbon HDMI cable. In this test, it again was extremely difficult to tell any difference, but I do think there is just a slight increase in detail and definition of instruments, as well as a bit smoother sound and sense of sound stage. Now, Audioquest touts as one of the qualities of the Carbon as "low jitter". How important is jitter in HDMI connections? Also, could it be that more expensive HDMI cables might be more likely to demonstrate a noticeable improvement in audio than in video performance?
Finally, I have not conducted it yet, but my final test will be to compare the two cables when playing higher resolution signals such as SACD, DVD-Audio, and Blu-ray movies that have DTS-HD Master Audio or Dolby TrueHD tracks.
Any observations much appreciated.