Sean, there are three types of mid level speaker cables that I tried along with the 12 gauge wire -- MIT T2 biwire and single wire, AudioQuest Slate, Nordost Flatline. The MIT single wire and AudioQuest did not sound appreciably better than the 12 gauge, and I found the Nordost and MIT biwire to actually sound a bit poorer (I'm suspicious that there might have been something amiss with the MIT cable, or something strange in the biwire configuration of the speakers, so the comparison may not be entirely fair). For reference, I'm running CJ amps into ProAc speakers, and the cable runs were around 12-15 feet each. Also, since my testing wasn't terribly scientific, I chose to dismiss any differences that were not very apparent and pretty dramatic.
As I mentioned before, my inability to hear, or my system's inability to produce a difference in the speaker cables may be entirely unique to me, and the results may be quite different for other folks. Also, I have not tried any of the higher end cables, and its possible that the differences at that level might be more striking. However, since I'm already getting a great deal of pleasure from the sound as it is, I'm more than satisfied to live with my perceptions (or delusions, you be the judge...).
Cheers,
Ken