Hello, I am running most things on ultra apart from the sky , 200% screen resolution, 1080p. I got an i7-6700 paired with a 3080 and I can get about 90fps once the train starts moving outside of a busy station. Stations can get as bad as 30 fps though. I am about to pair the new 3080 with a better CPU, can I expect some decent gains? I can get an i7-13700 for 325€, would that be a good option?
Not really. No matter what modern hardware you may have, busy stations and areas with lots of assets will give you low fps no matter what. I also have a modern gpu and a decent cpu and busy areas will give me 30fps or even lower.
It all comes down to how you configure your settings. I used to have terrible FPS But everyday play with graphic settings More Reshade I'm ready to go. There's a deputy here with a 4090 GPU, and he was still complaining about the SPF.
I’ve just gone for that (an i7-13700) with a RTX 4070 and I’m looking forward to experimenting My RTX 2080 super / i7-9770k was starting to show its age, especially with Niddertalbahn
As explained in this post, you should use Nvidia's DSR (Dynamic Super Resolution) instead of the in-game screen resolution increase. I can confirm that the quality is similar, but you no longer lose FPS.