I haven't studied the numbers, but I agree that the extruder motor is moving faster than X/Y/Z so a higher impedance seems logical. After pondering this for a while, I now somewhat regret referencing the reprap thread. I think its concept of applying the z=2*pi*frequency*inductance formula to stepper motors is debatable. The formula applies to sine wave signals. For square wave PWM signals like we're dealing with here, the frequency content is really in the rise and fall times of the PWM signal more than the pulse rate. ie, sharper square waves have more higher frequency content. But what do I know - my expertise was in digital circuit design, not motors!