For highs I intend the frequencies above which directivity plays a role in the measured FR and a lumped EQ is not applicable anymore. Broadly speaking, I think it should be around 2 kHz, if I remember some of the graphs I've seen in the past correctly.
1 kHz is probably still in the direction independent area, which I think (but don't quote me on this) can be corrected universally by EQ, at least under perfect seal conditions (which is like saying that practically speaking the distributed acoustic impedance behaviour could apply to most of the audio range, especially with HPs with poor sealing).
By directivity, what exactly do you mean ? Changes during spatial averaging when measuring headphones on a test rig ? HRTF ?
I'm trying to understand what you have in mind in the context, for example, of HRTF measurements being mostly done at the ear canal entrance, which seems to be sufficient to gather most direction dependent information up to at least around 7 kHz according to a few articles I've read on the subject, or in the context of headphones still operating under pressure chamber conditions up to a few kHz at least, unless I'm mistaken.