Make sure you are using the in-game calculation if you set the sensitivity there, is this the issue maybe? WPS don't affect it, and there doesn't seem to be a smoothing issue. But if you are using a polling rate above 1000 Hz and have slow movement, it can be a good idea to try 1000 Hz.
I've update the game with new FOV range, new FOV for ADS and fixed the sensitivity calculation to be spot on (it was off by about 0.02%).