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

No matter what the voltage is at your outlet, the idea is to get as much of it to your gear as possible, so the age-old counterpoint/argument "what about all the wiring between the power plant and your house?" is moot.
Voltage drop, and the corresponding performance loss that results from it, is real and it’s scientifically measurable. Unfortunately, every power cord suffers from it, but to widely varying degrees - up to a 3% voltage drop is considered "acceptable" in power cord manufacturing.
If your amp is only fed 117 volts, it will not produce the same amount of power/performance as when it is fed 120 volts. Simple.

With that said, you certainly don’t need a power cable costing 4 or 5 figures to combat/minimize voltage drop.
Is there snake oil out there? Of course there is! How else can a company justify a $10,000+ power cord?
However, the mere existence of snake oil does not negate the existence of voltage drop and its effects.

Try some different cables with different gauges and go from there.

Cheers and good luck!

Voltage drop... if it's a concern for you.

Have at it.

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=5.211&voltag...

Nordost Vishnu on my CD player, network streamer and preamp give me improvements I would say they were  greater than 5%. On my bel canto ref 500m mono blocks, Nordost Vishnu and heimdall 2s gave a slight improvement, I’d say less than 5%. The heimdall 2s were better, but not by much. 

Thanks guys ! So much truth in this thread ;-) !

I’m having trouble seeing how a power cord could make a difference.  After all, the stock cable should be capable of delivering the full mains power available to the amp, and the power supply on the amp is going to clean up any minor amount of interference that could possibly sneak in between the wall and the unit.  

I've seen a power cord cause a loss of about 40 watts on a 140 watt amp. It was easy to see why- there was a voltage drop across the cord. I measured it with regular Digital VoltMeter (DVM); its not rocket science.

By contrast, the romex in the wall of most houses is much better performance, being made of solid core copper. But this material is illegal for use as a power cord; so yes, that last 6 feet can make a difference.

Here is an additional issue, that I think will affect switching amps more than other types of amps. The voltage drop is not limited to 60 hz. The problem is that there is a power supply in the amp that is made of a transformer, rectifier(s) and filter caps. The rectifier converts the AC from the transformer to DC and the filter caps store that voltage for use by the amp. In-between peaks of the AC power waveform, the caps get discharged. The rectifiers only conduct when the cap voltage is lower than that of the transformer; if the caps are only slightly discharged (the normal state of affairs) then the time that the rectifiers turn on is very slight, perhaps a few milliseconds or even microseconds. At that time the current draw might be profound; if the power cord does not have the bandwidth, the pulses charging the caps will be current limited.

This can be bad for a switching amp, which needs a really clean power supply to sound right, even if they don't seem to be drawing much power from it at any given time. So its worth it to try a power cord that has greater bandwidth (which romex already has...).