"You can expect a 15% to 20% improvement in sound for each level as you move up the line.
I think you are misunderstanding what they are saying. I think they are saying that there is a 15-20% difference between each level of cable. Not a 15-20% improvement in your system sound but a 15-20% change in relation to the small change a cable makes. So 15-20% to the 1% change (SWAG) a cable makes.