In relation to achieving optimal sound
quality, is there any advantage to
pairing high-impedance headphones with
an amplifier for listening on an MP3
player or laptop, as opposed to just
using low-impedance headphones?
Yes, there is. In fact, there is an advantage to using an external amplifier for any headphones.
If you are resourceful, you could try building a CMOY amp in an Altoids box, like the picture below. The sound is awesome, and the cool factor is off the scale.
Why do amps like this improve the sound? I could talk about slew rate, total harmonic distortion and a whole host of other factors, but the simple fact is that having a booster amp like this means that your original amplifier doesn't have to work as hard. Most small audio output (including the headphone amps in the vast majority of computers and laptops) only produce about 100 milliwatts of power, and that's just not enough to insure high fidelity.
Here's how impedance affects the performance of headphones: a lower impedance allows headphones to capture more power from the amplifier. All other things being equal, they are going to be louder than headphones with a higher impedance, because higher impedance requires a higher voltage level from the amplifier to achieve the same relative loudness. This is why earbuds typically have a much lower impedance than other headphones; it is necessary to get adequate volume, given the low voltages provided by batteries.