First off let me state that I design PA systems for a living so im very skeptical about any of this cable and interconnet stuff. Having said that the following has just happened to me
1 - Tried a $250 interconnect between CD and amp instead of my Radio shack $20 Monster and the system performance dipped, so back to the Radio Shack cable.
2 - Tried a $150 pair of speaker interconnects over my $10 Radio shack monster zip cables, these totally changed the performance of my system for the better.
3 - Being logical I assumed the diffference was due to the greater AWG of the new cables allowing better current flow to the speakers so I wired up the speakers with some pure copper 8 AWG wire. The sound was slightly improved on the monster but nowhere near the improvement of the $150 Vampire speaker cables.
The conclusion ive drawn is that cable definetly have an impact on system performance, now of course to what extent you need togoto is another matter.