From what I understand, high impedance headphones came about in old studios. The main advantage is that you can run many headphones in parallel (on tube amps). I guess it would all come down to power transfer, the amp acts as a load in itself in a way, and if the headphone impedance is really low, you won't get a good signal as most of the power will be dissipated in the amp.
But most modern D/A end with an opamp, or at least I'm guessing the one in the macs do. It's certainly not tube. As much as opamps probably don't have the sound quality of tube, their output impedance is very low. So there shouldn't be a power transfer problem. So I'm guessing that noise shouldn't be a big problem. Sure the voltage in the high impedance will be higher, thus there should be a bit less noise, but really, that's nothing compared to having a cheap D/A stuck inside a computer. Also there shouldn't be a problem with the frequency response.
From what I understand, tube amps have a higher output impedance, so they can have a hard time with certain low impedance headphones.
Apparently, there's could be a difference in sound as there's a bigger magnetic field, so possibly better control over the diaphragm. But I somehow doubt that high impedance headphones will make a macbook pro sound magically good.
I could be totally be off, I never came across high impedance headphones. Kind of a cool thing to think about.
