It depends upon the measurement method on whether to apply "HRTF" or not. Most folks over at HF, when talk of HRTF was all the rage, seem to have confused HRTF with someone much simplier: FR compensation. I guess HRTF in its most simplistic form is a base compensation. An ideal behind HRTF is that the transfer function (I bet 99% of HF has no idea what a transfer function is in mathematical terms) should be three dimensional. One way in implementing this is to use individual impulse response transfer functions based on specific angular measurements in a spherical coordinate system. In practice, one would take limited set of measurements of different azimuth and altitude, and interpolate. The idea behind this is of course to to simulate a room / venue or provide directional cues with limited number of speakers. That's key. How HRTF got misapplied to the concept of very basic FR compensation, I do not know. Probably because it sounds fancy, and if one uses fancy words or acronyms, then one must be smart.

IEMs require a compensation to boost the a narrow area around 2-3kHz. When I spoke with UE, it was their opinion that the pinna boosted that region, so compensation was required for IEMs, which of course bypass the pinna. I do feel the same way in a subjective sense. (Note that the IEM measurements on this site already compensate for this - so the measurements you see are "perceptual" rather than "raw" measurements.) The raw measurements of the UERM do show a boost in this area. The ER4 also have a boost, although the I think it's overdone. IEMs which don't have that boost in the raw measurements seem to sound too laid back with vocals, particularly female vocals. This is not necessary a bad thing. The "BBC curve" used for speaker voicing pushed 3k back to lessen exaggerated room effects from the combination of the recording venue and the room used for playback. I find the BBC curve useful for live concerts, but not so helpful with studio recordings.

Speaker measurements are done with a microphone in free air. The v1 headphones measurements on this site where done in a simulated free-air environment using a porous sponge. These measurements were combobulated with another sealed plate measurement to arrive at the correct bass.

As for compensation used here, it's rather simple. I compare to references I consider neutral: my speakers and my audiophile desktop setup (both surgically EQ'd via digital PEQ to a target FR with a slight downward slope. If no one wants to believe me that this curve yields the "best" most neutral results, just ask the Harmon researcher Dr. Sean Olive, PhD.) I think I've posted a few listening position FR graphs of those setups. People can argue all they want about HRTF, compensation curves, diffuse field, AES papers, B&K recommended curves, etc. but at least I've provided a reference - and compensated as best as I could the measurements on this site to those references. Again, the measurements here are perceptual FR graphs. And finally, several folks here have actually heard my reference setups.

But yeah, in the context of how most folks at HF use the term HRTF with measurements, it's BS.

Actual HRTF is a 3D complex domain function, but in terms of headphone and IEM listening, only the 2D component matters, and usually only the one for 60 degrees to the side or front one, depending on which one is chosen.
The rest matters for spatializers or head tracking.
It might be that Fourier model of the head function is insufficient as the function is actually nonlinear.
Head itself is a minor part of the HRTF - the most important part is the outer ear.

Mono equalization by ear derives the frontal HRTF convolved with headphone/IEM response. Usually this is very close to the compensated measurement, perhaps with some resonant shifts and minor differences in upper midrange gain (around 3kHz) as well as ear canal resonance amount (around 6kHz).

There are still a few variables in headphones - distance from eardrum, central or off-center positioning of the driver which changes outer ear gain.
For IEMs, there's insertion depth and exact outer ear gain.

Both can also suffer from bad acoustic seal, like this:

In the overall scheme of things, I just like to keep things simple rather than worry about HRTF and other nonsense. In other words, how do the HD800s or <insert headphone here> sound different in relation to the HD650  or <insert other headphone here>, and do the measurements jive?

Those searching for the holy grail of absolute measurement perfection citing a myriad of authorities, methods, etc. are doomed to fail, or at the very least are not seeing the forest for the trees. It's like those fuckers looking for the "God" particle. What a fucking waste of human thought, time and money. What would be funny if after they "discovered" the God particle, some other random fucker physicist hypothesizes about a "Super-God" particle. And you know that's going to happen.

Exactly. While speakers can measure neutral according to the Stereophile method of using anechoic measurements - hanging the speaker up in the air and using a measurement mic 1 meter away - there's no guarantee they will sound even neutralish when in placed in most real rooms. This thing's anechoic measurement http://www.stereophile.com/content/sonus-faber-stradivari-homage-loudspeaker-measurements a was fucking god-awful bass monster in the small hotel room at THE SHOW. Anax I and quickly ran from the room in horror. But I bet in an auditorium sized room placed far far away from any walls, it would sound decent (still too much bass for my tastes).

Many studios pros and even Dr. Sean Olive (see Tyll's IF article) have described a modified curve (this modified speaker-in-room curve is essentially a tilted straight line with the bass end about 10dB above the treble end) at the listening position as being most preferred. It should be noted that I only brought in Sean Olive because some people worship him and require an authority. Literature on studio monitor EQ has long suggested this curve before Tyll's article mentioning Dr. Olive's research.

Even then, I personally use about a 7db curve (final curve is tuned by ear) with my references, possibly because my living room and work room are more reverberant than most studio environments. At the end of the day, at least a few speaker manufacturers do know what they are doing. There were four / five setups at THE SHOW 2013 which I felt were within reasonable bounds of neutrality / listenability and I didn't really hear anything which sounded nasty like an Ultrasone or HD700 except for this wide baffle VoaAtiv speaker which was a tonal balance disaster: