Thanks for the input. I applied what you recommended and did new benchmarks using the version from SK_Odyssey_slow_stream.7z.

Sadly I wasn't able to accomplish any decent improvements.

SK_Odyssey_slow_stream-Mod with Flip Model, 3 backbuffers, 4 max device latency, 1 presentation interval. (I also had to set Fullscreen=true, ForceFullscreen=true and OverrideRes=1920x1080 in dxgi.ini for it to work properly.)
(values in () give the difference to the Special K-benchmark from above)
FPS min: 17 (+4)
FPS avg 39 (+2)
FPS max: 57 (+3)
CPU min: 17 ms (+2)
CPU avg: 26 ms (-1)
CPU max: 59 ms (-17)
GPU min: 13 ms (+/- 0)
GPU avg: 21 ms (+/- 0)
GPU max: 53 ms (-12)

For comparison, another benchmark using DXVK:
FPS min: 28
FPS avg 72
FPS max: 107
CPU min: 9 ms
CPU avg: 14 ms
CPU max: 36 ms
GPU min: 9 ms
GPU avg: 14 ms
GPU max: 33 ms

I just made an interesting discovery regarding Assassins Creed Odyssey I thought I should share and that might heavily help all those that were in false hope Special K Mod would fix their performance issues.
I did some benchmarks with the games internal benchmark tool for that. To make sure it was fault free, I repeated them all a couple of times.

System Specs I benchmarked with:
i7-4790K
RTX 2080 Ti
32 GiByte of RAM

No Modifications:
FPS min: 25
FPS avg 42
FPS max: 60
CPU min: 17 ms
CPU avg: 24 ms
CPU max: 39 ms
GPU min: 13 ms
GPU avg: 20 ms
GPU max: 33 ms

Special K Mod (taken from SK_Odyssey_fast_stream-v0.7z, the most recent Special K makes the game crash at startup)
FPS min: 13
FPS avg 37
FPS max: 54
CPU min: 19 ms
CPU avg: 27 ms
CPU max: 76 ms
GPU min: 13 ms
GPU avg: 21 ms
GPU max: 65 ms

So far this might not be anything new to us as Kaldaien stopped supporting Ubisoft Games for a very decent reason.

Now there is a tool called DXVK, which also uses dxgi.dll-Interception, like Special K, but is actually a wrapper from DirectX to Vulkan. The primary goal of the DXVK-Project is to make games using DirectX run on Linux. It's feature to also work on windows is just a nice side effect of that.
Devs of DXVK stated they couldn't think of a scenario where DXVK could improve performance on Windows over the native implementation of DirectX.
Yet, I was able to absolutely make sure I got heavy performance improvements by using DXVK on Assassin's Creed Odyssey.

DXVK:
FPS min: 32
FPS avg 73
FPS max: 104
CPU min: 10 ms
CPU avg: 14 ms
CPU max: 31 ms
GPU min: 9 ms
GPU avg: 14 ms
GPU max: 32 ms

As stated above I repeated these benchmarks a couple of times in each configuration to verify this effect.
I assume the improvements might heavily depend on the CPU, which was kind of bottlenecked by my aging i7-4790K and no mod.
Now I'd love to see if some users could verify my discovery by doing some benchmarks too.
If you do so, please pay attention to the fact that DXVK does lag for a very short period of time when it has to render shaders for the first time to create it's "ACOdyssey.dxvk-cache"-File. This initial lag requires you to repeat the benchmark to get valid results.

I hope Kaldaien won't mind me to create this thread in his forum, but yet it's a good chance to put more shame on Ubisoft, which I hope will be fine by him. ¯\_(ツ)_/¯