I would have thought the area right between 30 and 60 would be most interesting. With 40fps VRR being a thing, which I have yet to see if that is actually okay or good as some think it is. 30 is objectively just gross for games that are supposed to look fhd or 4k clear, while it was never a problem for me with PS3 games, still isn't. Games that are blurry anywy aren't destroyed by the inherent 30fps motion blur. I have not really tested anything beyond 60 properly. My monitor and TV does support it but since my PC is not top notch, achieving something much above 60 is not possible unless reducing details much and I am not sure any console game I played supports modes above 60.
I would assume 90 might still be noticeable though, maybe more in when going back to something slower, I guess, diminishing return should definitely set in then, since 120 and above require either impossible beasts of machines or reducing details quite significantly, much more noticeable than some minor more smoothness. I am certainly no Pro CS gamer that values that that much.