going by experience n not theory, dropping the rear sprocket from 39T to 33T (leave the front sprocket alone), gave me a much smoother ride on the highways with less vibration at top speed.
downside is a little slower off at the lights n less pulling power with pillion uphill.
no problem cos i normally ride solo.
also in town, i tend to use 3rd gear most of the time, tat's how smooth the kapcai is with a smaller rear sprocket.