Planning to set up another server, which OS do you recommend using? Linux (which one) or Windows? 

Just noticed these servers https://www.battlemetrics.com/servers/rust?sort=score&q=paranoid
having 600-2000 server fps with 30-80 active players, how's that possible? Low tickrate etc maybe? 

That high of FPS is just a waste of resources and fluff, it won't provide you any benefit over normal, reasonable rates.

I'm aware, but the benchmark says something about performance and if it's able to handle high number of players/entities.