That the G400s has smoothing is right.
No, not particularly, optical Sensors(which includes Avago´s LED and Laser illuminated ones as operation is basically the same) have a certain "Base CPI" due to its Image Array from where higher CPI are getting upscaled, working with less Data than actually needed causes less Precision and Jitter(jerky movements) at higher CPI, so to counter that smoothing was introduced, which in a way works like Vsync for the Mousecursor.
My Suggestion would be to get a G402(because it is lighter than the G502) from a shop with good return policy to try it out yourself.