The mean age of an ocean basin is inversely related to spreading rate: slower spreding increases the mean age, thus there is more subsidence, and sea level falls.  How much?  The mean subsidence wbar of a basin of width W with spreading rate u is:

wbar = (((∫_0^(W/u))A Sqrt[t] \[DifferentialD]t)/((∫_0^(W/u))1 \[DifferentialD]t))

Simplify[%]

Using km as the distance unit and My as the time unit, we can first compute the subsidence for a basin 6000 km wide as a function of spreading rate

subsidence  =  
      wbar  /.  {A -> 0.35, W -> 6000}

\(Plot[
      subsidence, {u, 20, 
        200}, {AxesLabel\  -> \ {"spreading rate", "subsidence"}}]\)

and then calculate the change in mean subsidence brought about by a factor of two decrease in spreading rate

(subsidence  /.  u -> 120 ) -  (subsidence  /.  
          u ->  60)

This change occurs over a time scale ~100 My so that the rate of change is quite small on human time scales (less than a millimeter per century)