Has anyone purchased upgraded cables for their TV and actually noticed a difference from before?

I have a Pioneer Elite Plasma TV, and a HDV cable box. I am wondering would I actually notice much improvement over what I already believe is a great picture on my set

It really depends on the equipment you have, and the cables you already have. The rule I like to use and you will be successful is 10% of total system cost should be in cables.

A good connection does not degrade a signal, but cables by there very nature do this, cable designers strive to not add capacitance, or impedance to a cable, they try to counteract skin effect, audio signal propagation issues, etc.

But it is again all about your equipment. The 10% ratio works on almost every set up I have done. But, Most of the cables you have readily are for mid range products.

Audition the cables, meaning get them, let them break in for a few hours and see if you can tell the difference. Any qualified sales person or tech will be able to show you what to look for. Way back in the day when I was still selling AV, I would hook up both, preferably side by side to point out the difference in cables, some albeit just a few, preferred what I thought was a bad picture, but I let them (since it was for them, not me) make up their own mind.

You should have this kind of mind set when pursuing a improved picture. If you don't think the image or sound is improved then don't keep them.

Now about diminishing returns.

I will use an analogy to make it easier to understand.