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

It looks like increased prop diameter helps climb, but not as much as
weight decrease or more power.
Looks promising..
Perry

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