This assumption assumes that the power to the USB converter is good and not dependent on the cable or USB port.
Power on a USB port is never high-quality, therefore improving the power feed to a device that is powered from the USB port may help.
That means as I understand it, that improving/upgrading the USB cable would not improve the power supply as the USB port has poor quality power to start with. I am thinking about standard type USB cables without external power supply and asynchronous DAC's, Transports.
Scenario1: If the async usb converter doesn't draw power from the usb port (like his Pace car), then the usb cable doesn't matter
Scenario 2: if the usb converter draws its power from the usb port (such as the stock hiface), then the usb cable might impact the sonic performance.
In scenario 2 a good usb extender cable can help get rid of some high frequency distortion in the usb power lines. If the usb cable is set to act as a low pass filter, it can improve (a little bit) the power from the usb port. Of course, such a low pass filter won't get rid of all the "junk" (ripples, spikes, ...) but it could at least in theory get rid of the high frequency content.
To my knowledge, there at least 2 cable companies that apply that concept for their power cables: Wireworld and Hifi Cables & Cie (a small French mannufacturer). I am currently using Hifi Cables & Cie's Powertrans Plus cable that apply the principle of increasing the capacitance (among other things) in order to filter out some of the junk in the power supply.
The result is that the Powertrans Plus power cord works very well with all equipments (it is not system dependant). It provided improvement for different equipement I tried it on: DACs, headphone amps, big tube power amps, powered subwoofers... and even for my TV set.
That it is to say that a good usb cable extender should improve (to some extent) the performance of the stock hiface.
Hiface, jkeny's idea is more elegant. Instead of trying to cure the usb power supply, one can just bypass the usb power from the computer and use an external 5V power supply.
I wonder why nobody has thought before of building such an "audiophile" usb cable. Such a cable would not only work with the hiface but with any usb converter that draws its power from the usb port.