The way I understand what Steve said is slightly different:
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.
A custom cable that breaks-out right before the HiFace and adds bulk capacitance and HF capacitance (.1uFd) might help a lot.
|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.|