Yes people can see the difference between 20 and 60fps, but it has a lot to do with the resolution, detail, motion, and size of screen. It is EMMENSELY important in flight sims, especially combat ones. It also seriously annoys me when people post false information on that topic.

 

Does locking the framerate produce a more stable low framerate, or is it just for V-syncing if you have very high framerates to begin with and you want to eliminate frame tearing? Because v-sync usually reduces framerates by about half, rather than produce a more consistent average at the expense of peaks. In constrast, DCS Blackshark has a frame limiter that is capable of producing more consistent averages with only a reduction of the peaks, rather than a whole halving of the averages to get frame syncing with the monitor. Which is FS2004 doing?

i get 70(on ground max detail) and 100-200fps(in the sky)

I get 30 because its locked there in NI and it never changes or stutters.