The variations are enough to make one HRTF incompatible with another's HRTF to the extent that true 3D imaging is not believable. I believe that could be found in literature. And I don't think that peaks/dips necessarily stay at the same frequency. It varies with all the other variables that are out of control in that scenario, results from completely to not as much. Elevation perception is one of the more fragile things to translate. It's worse than just an error, it can completely collapse. Look, this gets at why binaural, and especially binaural on speakers, has never really worked universally, which is part of why it has not been generally accepted (a small part, but none the less). Capture has to be done with a fixed generic HRTF, and then it's played into specific and variable HRTFs. DSP translation between HRTFs is non-trivial. There are many vectors that image well using a generic HRTF, like behind and slightly above, but frontal elevation, particularly that of a dead-center source, is really difficult. That's largely because humans don't perceive frontal elevation well to being with without visual support.