If the manufacturer put a $300 power cord in the box and increase the list price by $300, as a customer, would you like to pay for it without a choice? There are so many power cords in this world, and all have their own character. So, which one do they force you to buy? By the way, $300 is only a low to mid-grade cable!!!
Many people say they are not that good with their ears and don't believe they can hear the different.
Let me tell you. If you are on the phone with your buddy and you can hear whether he/she is happy, sad, tired, angry, sick .... then your ears will be able to pick up the different when you try different "good" cables on a "good" system.
If you talk to you other half for 20 seconds on the phone and still ask who I am talking to, then forget about it.
If you ask if a cable worth that much, then it will be a totally different question. So, for the original question, cable upgrades are real, rip off.... may be.
Go for a test hear yourself.