The flex riser from LiHeat arrived, and damn is it a fine product!
The riser is not only shielded but also nicely sleeved, which gives the whole cable a really nice look and feel.
This is the approximate configuration I'll run it in. You'll notice there is a small bulge in the middle. That is due to the cable consisting of somewhere between three and five layers internally that all have the same length. Because of the bends it does, the inner shielding is on a shorter way, so it has to compensate by bulging. (I hope that was an understandable explanation) Not a big issue, though, I have more than 13mm below the GPU.
The bend radius is also quite sharp, the cable is about 2.3mm thick and can easily do a 180 in just 5, maybe 6mms of space. This means that my solution for the GPU mount is feasible and I could even scrape 5mms off the case height at the bottom. This is a huge advantage over straight ended risers like the 3M one which would require me to make the case 20+mms higher.
I also tested the distance the PSU has to have from the motherboard and the one used in my CAD model is perfectly fine, could even reduce it a little bit.
I installed it into Freilite Alpha and am writing this on that rig right now.
Additionally, I tested the performance non-scientifically with GTAV, and still got smooth 60FPS in 1080p as always. (Most settings on very high, some on high, density and variety settings on maximum, no AA, 16x AF)
I will have to order a custom made ~360mm long riser for the final thing, but right now I just wanted to assess whether everything fits, which it fortunately does.
There were connectivity issues when installing the riser first, but that was quite probably caused by the way I bent the riser (very close to the solder points, and LiHeat does offer "armoured" versions of their risers that prevent that sort of thing. Once it ran, it ran really well, though. The connection is well made, even waving the GPU around, bending the riser like crazy and wiggling on the connections didn't change a thing. Very solid, I love it.