An upgraded power cord can make "less of a difference"?
In my experience and in every system I've heard one used,there was always a difference for the positive.
There will always be a difference, how can there not?
Every power cord , stock, DIY or expensive name brand has a different contruction and has been used for different periods of time,which could be beneficial or detrimental ,depending on which side of the burn in/worn out side of the fence you're perched on.
I suppose you can make the same statement with all things in audio?
So why bother with tube rolling or capapcitor upgrades if doing so will only make more or less of a difference?
If what you have been using only makes "less of a difference", then perhaps you should try a little harder and broaden your search.
The good stuff is out there.
And besides just being different, what you'll hear will be an improvement.
My experience,and that of all "others whom I know" who have done the same.
Peace on earth my friend.