The measurements are probably comparable, at least for the most part. However, reading a response graph is not as straightforward, especially for headphones. For example, nearly all of the high frequency aberrations (peaks and dips) are usually not due to the headphone driver, but instead due to the reflection(s)/resonance(s) between the [relatively] reflective ear surface and interior of the headphone. In addition, the headroom graphs are made on a dummy head that replicates the average human ear and ear canal structure, and the measurements include the resonances that occur as a result. These same resonances also occur when you listen to speakers, but since a dummy head is not typically used to make speaker measurements, you would not normally see the effect(s) of such on the final response graph(s). Also, the headphone driver is directly against your ear, and a speaker for example, would be located in front of or near in front of you off to the side somewhat, and a high frequency reduction occurs to some extent with the speaker if you measured from the perspective of inside your ear canal vs. a free-standing measurement microphone. The end result is that one can not directly interpret the frequency response graph of a headphone as to what one would expect to hear from a speaker in free field space with a similar looking esponse trace.
-Chris
