Impedance has to do with how the guitar interacts with the first thing it's plugged into. In my case, I'm plugged into a wireless transmitter that digitizes the signal and that's where the interaction stops.
Try to change input impedance settings with the Fuzz Face first after the input block. You'll hear the dropout due to the option change but no tonal difference. Placebo likely. Try that again with a quarter inch cable in place of the wireless system and the difference will be stark.