Can a power cord increase the resolution of an Class D (SMPS) amp by more than 5% ?

Absolutely, some much better than others. In the past I have used top cords from TekLine, but have mostly switched to Wire World, who calls his 'power conditioners'

They certainly increased the overall performance of my 2 Emerald Physics EP 100.2 SEs

I m using WireWorld power cords. He calls them PowerConditioners. They absolutely work, but how much better depends on your system