The A.C. power that's delivered to your home is generated dozens and hundreds of miles away. Between the generation point and your home it's converted up, down and sideways multiple times. Finally, on a pole or box near your home, it's converted One Last Time, a "neutral" line is added, and the drop to your home is made. The feed enters your home, goes to a distribution panel (aka: "breaker box" or "fuse box"), then, from there, to the various outlets and light fixtures in your home.
What these tweak power cord manufacturers would have you believe is that, after traveling all that way, somehow a Special Last Six Feet of Premium Power Cord is going to somehow make the electricity "better."
As to whether or not you'll perceive an improvement after buying and installing an expensive, "high-end" power cord: Of course you will. Just like your car always seems to run a bit better right after you've given it a good wash.