Good question. My understanding is that when inserted, the tube basically acts like an extension of the ear canal and what matters in terms of the ear canal resonance frequency is that total length of tube + canal.
You inspired me to do some measuring of the VK4 using various tips. I took 3 measurements with each of these tips:
1. Stock medium (small was too small to get a seal on the coupler)
2. Xelastec medium (hard to get a consistent insertion depth because my smallest xelastecs are a little too large for the coupler)
3. MH755 small and medium (both seal well but get different insertion depths)
4. Cheap foam tips
All below results are smoothed to 1/48 octave to remove squiggles.
Each tip was measured 3 times, removing and reinserting between each measurement.
First a comparison of the averages of each tip:
View attachment 135325
Then a look at the amount of variability just for the Xelastec tip
View attachment 135326
And lastly a focused comparison of MH755 small and medium:
View attachment 135328
There are definitely differences in the treble, though based on what I see with the Xelastec tip and the MH755 small vs medium, those differences may largely be due to differences in insertion. The one thing that does seem to be a clear outlier is that the stock tip (which I think has the hardest silicone) has more of a 16 KHz peak than any of the other tips, and the foam tip is by far the most subdued at 16 KHz. So perhaps material differences do make a bit of a difference with reflection/absorption in the very high treble? That said, I believe these IEC couplers are not particularly accurate in the highest treble frequencies, so take that with a grain of salt.
EDIT - One other observation, the MH755 tip comparison shows how deeper insertion moves the ear canal resonance up in frequency and down in level, so getting the right fit and insertion is definitely important, regardless of which tip you use.