....For a US$1.100,- more, the I-mac Pro does not seem to offer much more than just the (cool) darker colour and obvious evidence (for those around to you) that you, in fact, got the Pro!...Is there anything obvious I am missing here??...
What you're missing is that significant performance gains exist for certain workflows. For others it's less so.
It's about knowing what your workload characteristics are and how this maps to (a) The iMac Pro hardware when (b) Using your specific software and tasks.
The iMac Pro is a lot quieter under sustained high-CPU loads. For all the people who were forced into an i5 iMac for noise or temperature reasons, the choice is NOT between an i7 iMac vs the iMP but the i5 iMac vs the iMP.
Mac Performance Guide did extensive tests, mostly focusing on Photoshop and still-image processing, which generally found limited benefits from the iMac Pro -- NOT because it wasn't fast but Adobe's software didn't take advantage of the hardware. In general he found a top-spec 2017 iMac is often the best choice over an 8 or 10-core iMP -- for his particular workload:
https://macperformanceguide.com/iMacPro_2017-Introduction.html
However I've personally tested my 2017 top-spec iMac vs an 8-core Vega 56 iMP on FCPX, and there are significant (but not universal) gains for the iMP.
Test file: 6 min 9 sec UHD 4k H264 29.97 fps 100 mbps 4.4GB XAVC-S file from Sony A7RII. In all cases the file was imported with "leave files in place". The proxy files were deleted after the transcoding test. All other tests used the on-disk camera file not a transcoded or re-wrapped version.
Create proxy (all times min:sec)
iMac: 1:22
iMP: 1:48
Export to 1080p H264 Faster Encode:
iMac: 3:33
iMP: 2:26
Export to 1080p H264 Best Quality:
iMac: 6:59
iMP: 4:43
Export to 4k H264 Faster Encode:
iMac: 3:36
iMP: 4:30
Export to 4k ProRes 422:
iMac: 3:45
iMP: 2:28
Timeline render tests (via CTRL+R):
Aged film effect + default sharpen:
iMac: 5:31
iMP: 4:03
Stabilize 15 sec of 4k clip:
iMac, analysis phase: 0:45
iMP, analysis phases: 0:36
iMac, render phase: 0:05.6
iMP, render phase: 0:03.9