Dude, it is very easy to test whether the USB cable makes a difference or not: an inversion test! fryingpan
Setup:
Play the same audio-clip on two tracks, one of which is inverted to the other.
Send one track to the Master sum of your DAW.
Send the other track directly to an output of the Fireface UC and route that track back to an input via Totalmix' routing option.
Create a third track that monitors the input of the routed track and compensate for any latency/delay (i.e. in Live 7 I have to manually compensate by 122 samples at 32 samples audio buffer setting).
The resulting sum on the hosts Master bus null *perfectly*, and I don't mean with noise lower than -144 dB, but *completely* (aka no signal present).
By the way: Totalmix' routing does *not* happen in the driver, but in the Fireface UC, so the signal passes out of the host over the USB cable into the UC box, is routed in hardware and then passes back over the USB cable into the host.
PS: Does anyone know why (in Live 7) manual delay compensation has to be set to 58 samples + 2 x audio buffer? Shouldn't that be handled by automatic delay compensation of the host?