I have connected a Russ Andrews high current power lead to my Thomson LCD TV and a YellO to the 17 Relisys. The answer is yes it certainly made a big difference in my case. Colours are brighter, the picture much more 3-D and less noise in the background. Initially I had the YellO on the 27 but when I fitted the high current power lead this made a noticeable improvement.
Mark Grant made my SKY+ power lead. I never complained about my SKY picture quality since. I am too tight to spend money on plasma power lead it's already miles better than anything I've seen in the shops.