The 160kmph rating is only for the FIAT bogies not for the coach as a whole. Because of the open windows, passenger comfort is severely affected at high speeds. (This is like the 'side window buffeting' seen in cars, where you can experience vibrations and sounds when driving at high speeds with the windows down). An absolute speed limit for comfort probably can't be decided, but since the sounds levels and dust levels increase exponentially with speed, 110-130 kmph is probably a safe limit for non-AC coaches. Also, the wind induced vibrations make it unsafe to run non-AC coaches at high speeds. I found the 120kmph number on the IRFCA page at the link below
click here
Also,...
more... I have attached a document I found online while searching for Vibrations and noise levels in Non-AC coaches. This is a letter regarding trials conducted to assess the effect of the train's speed on the noise and dust levels in the coaches.