To answer your question as directly as possible, the move from 8PR to 8TC may produce some subtle audible improvement, but don't expect anything dramatic. The main difference between these two cables is the dielectric (insulation), with the 8TC being coated with Teflon. I have owned 4PR, 8VS, 4TC, and 8TC, and can't reliably hear differences between them. (However, that may be due in large part of my aging ears, since I don't hear tones above 10kHz.)
If you want to upgrade your speaker cables, I recommend you do what I did when I upgraded from 8TC: buy Alpha-Core Goertz MI2 cables. There was a long thread here on Audiogon several months ago, with some excellent comments by our resident guru, Sean. Look in the archives for the following thread:
http://forum.audiogon.com/cgi-bin/fr.pl?fcabl&1093784344&read&keyw&zzgoertz
The Alpha-Core Goertz MI2 sells for about the same price as the 8TC, but in my opinion is a superior speaker cable. For more info on this cable, go to Alpha-Core's informative web site:
http://www.alphacore.com/goertz.htm
