14.5" diameter vs. 15" diameter
 

If there was two props identical exept for the diameter, taking for instance 3 blade Cleaver, 28 pitch, what would the performance difference be on a 14.5" and 15"

Identical is a relative term.

14.5= 150 more rpm and quicker acceleration and less stern lift.

15= 150 less rpm and less acceleration and more stern lift. Possibly more speed depending upon the boats personality.

Brett