Here is my favorite plot of aircraft performance as
a function of various changes from Crawford's book.

Note that increased prop diameter does nothing for top speed.
Conversely a decreased prop diameter would not hurt top speed much.

An increased prop diameter really helps the climb however.

About a 10% increase is HP is roughly the equivalent of a 10% reduction
in drag as far as top speed is concerned. A 20% increase in HP
increased the top speed about 12 MPH in a stright line.
Little else hurts or helps the top speed in a straight line. Top speed 
in a 2G turn is another matter entirely. Not covered in this book. 

Paul Lamar
 
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