If you feel like modding, remove the back of the shell and stuff it full of damping material (in this case, mineral wool is best, as it attenuates most >5kHz) or preferably replace it with 1.5 cc of mineral wool covered by anything, e.g. lacquer.
This will fix the treble and slightly linearize the bass. Do not cover the front vent, this causes the bass to explode.
What will remain is the barely audible distortion and a hint of grain, which I think comes from the driver grille.
Pity they do not have better air, end at 17.5k. Darn, your new measurements are accurate even there.
GR07 mkII sounds identical. It just looks silver and the cable is silver-coated copper instead of pure copper, so it doesn't green anymore. Slightly higher impedance.
Also this: http://rinchoi.blogspot.com/2012/10/vsonic-gr07-mkii.htmlhttp://rinchoi.blogspot.com/2012/10/the-effect-of-ear-sleeves-vsonic-gr07.html
I found that the large VSonic single flanges (conical shape; not #1) work best. Duh. (Esp. if you mod it.)
Comply foams unfortunately rob the IEM of some air it desperately needs.
Long RE-272 biflange with an adapter (to reduce diameter to 3mm) works almost as well, boosts air while slightly attenuating 8k, but isn't as comfortable.
Adding an extra 1.5 cm of 4mm silicone tubing to simulate a longer ear canal reduces the 8k blip even more, but it breaks ergonomics.