Well, yes and no. You're confusing the issue. What you're referring to is generally quoted as how much sound is produced per watt (or milliwatt), at a given frequency, as measured by a sound pressure level meter, a set distance from the drivers. This value does not necessarily correlate with impedance. In other words, they are two different specifications.
As an example, let's look at the three different impedances offered by Beyerdynamic, for their DT 770 model:
Beyerdynamic DT770 / 250 ohm: 96 db @ 1mW / 500 Hz.
Beyerdynamic DT770 / 80 ohms: 96 db @ 1mW / 500 Hz
Beyerdynamic DT770 / 32 ohms: 96 db @ 1mW / 500 Hz
You will note that the manufacturer of this headphone lists the sensitivity
as exactly the same, for three different impedances. So the sound level produced by each of these cans is exactly the same, given a 1mW input at 500 Hz. Using your terminology, they are equally efficient. Of course, they don't specify what sort of amplifier they used to achieve that result. They also ignored the effect of frequency (though I give them props for including the frequency used in this measurement).
Using the points in your discussion, if you drive any of these headphones with an amplifier that is capable of providing the necessary voltage and current, then theoretically you ought to get the same sort of result. Without resorting to a lengthy explanation on ohms law and calculating impedance using imaginary numbers, let's just postulate that you get the same result, if and only if your amplifier can indeed match the right voltage and current to the load presented to it. I assert that the load for each of those headphones is different, notwithstanding their equal efficiency ratings. Each requires a different voltage and current to achieve the same power, and thus the same sound level output. But those are two different things.
So back to the original question: is there any advantage to higher impedance headphones? Let me modify my previous answer thusly:
If you can hear the difference, AND if you have an amplifier that can produce the necessary voltage and current needed, then higher impedance headphones are better.
If you can't hear the difference, AND/OR you do not have an amplifier that can produce the necessary voltage and current needed, then skip the higher impedance cans. Go with what you can drive with an ipod alone, and leave the arguments to people who write long messages late at night on mysterious internet forums...