lThe mass flow through a turbine is not affected by the down steam pressure due to the fact that the steam flow is choked in the throat of the different stages. Thus the increase in condenser pressure only affects the last stage, until that pressure is greater tha the pressure in the throat of the last stage of the turbine.

The velocity in the last stage would probably be affected by the condenser pressure and hence the adiabatic efficiency would change. I would like the typical relation ship between variaton in adiabatic efficiency of the last stage and condenset pressure.