Smoothness is to remove jitter if you're experiencing it. There's not really a PC equivalent. If you set smoothness too high, you'll end up in a place where the number of frames the XIM is smoothing out will negate the purpose of running at 1000hz.
Smoothness doesn't really have anything to do with curves.
If you get an Apex and you run at 1000Hz and experience jitter, you can increment smoothness slowly until you no longer have jitter. That's its purpose and the only way it should be used.