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.
The excellent www.RussAndrews.com website has documents that can be downloaded, you just need to register your details. One of them cover mains and goes into great depth on what to upgrade first.
The suggestion is to sort out mains first, including leads, extensions, main sockets and if you can afford it a new mains ring/spur.