Hmmmm , that´s a weird thing.
Do you only have that low FPS in COOP games or also on versus matches with just a few players only ?
Your display driver settings are ok and also your ingame graphics settings are on low.
There is a bottleneck somewhere. Difficult to find.
Did you already try to use MSI Afterburner to find hardware bottlenecks ?
It´s a freeware monitoring and overclocking tool which runs with any hardware.
In that installation package it also includes the Riva Tuner Statistics Server.
Install both and let your Hardware temperature and workload "from each CPU core" monitor on the screen
In the Riva Tuner you can choose the exact place on your monitor where to have that information.
You also can choose GPU temp/ workload , RAM workload , SWAP file size, Voltages, etc.
Also try the command -USEALLAVAILABLECORES in the startline of Sandstorm ( in Steam you can edit the startoption settings for your game )
For comparison i made screenshots from my settings in this guide i made :
Here is another Guide in the steam from to get a better game performance :
I know you wrote that you tried so many guides etc. but may be you didn´t have these.
Worth a try.
In the second guide there are some differences to mine ( PhysX setting in driver and Super sampling off in video settings ingame ) so may be that settings work better ?
I wanted to have maximum performance with a nice flicker free video quality.
What i didn´t like in Sandstorm were the flickering textures in the far distances.
The human eye reacts on any movement in the periphery and those flickering textures made me crazy thinking there are some bots moving but it was just bad graphics settings.
On my solution the picture quality is good enough and i have not a single flickering texture.
To achieve that i have super sampling on x1,25 ( ingame ) and edited the Engine.ini