ad 1: They do. I remember, it was in the late seventies- early eighties, how bowled over a dealer friend and I were, as a tweeker-inventor ( and snake oil peddler, as we first thought) walked in, hooked on his home brewed powercables to the dealer's systems and let them rip. As I remember, there was much more "presence" than before with his speciality cables. What had happened: The answer was simple: The wires were of bigger diameter than usual at that time and they were not screwed/clamped unto the plugs but very carefully soldered on. The dealer, who got the exclusive rights, from then on made a fortune with these power cords. He charged heavily for them, but got away with it, because the improvement in sound was definitely there, even for tin ears to hear. That was a few years before "the wire craze " hit our shores . In the meantime the market has progessed in marketing hype, snake oil but also in the technology of making speciality cables. However, to my ears at least as far as PC's are concerned, the law of diminishing returns vis a vis price comes in fairly early.
ad 2: ALL
ad 3: I don't really know which is "best", because most PC's I make myself. I've tried a few of the famous=expensive brands, but was not convinced. Perhaps it was my hearing. Don't know. The other day however, I bought a pair of WHALES here at Audiogon, actually intending to pass them on to a wire crazed audio friend of mine....and then most calously decided to keep them, because on the Spectral 360's they beat the s**t out of my homebrewed ones. So at the moment they are "the best". But who knows, they look so impressive in their nice fat roundness, I may be just delusional. (~;