Al,
In the attached picture, the 20B impeller (right) is .170" larger diameter than the 13B (left).  Note that the 13B in the picture is from an '89 n/a engine.  Not sure if 3rd gen or Cosmo impellers are the same size as the 20B.  No idea how much more coolant the 20B pump will flow than the smaller 13B pump.  Just a data point in case you have cooling issues.
Mark 

Mark;

My opinion would be that the .17 difference in diameter is negligible compared to other differences in the design (shape of housing exit flow, clearances, etc.)

Al