"Stan, the first thing that I
"Stan, the first thing that I have noticed is the high propeller slip in your boat. If you are running a 17" propeller at 5200 engine RPM and the gear ratio is 2.15 to 1 and you get only 25 knots, your propeller has a 35.3% slip, which is on the high side for a planing boat. It could be that your boat is too heavy, or has a fouled/rough bottom, or the propeller is spinning in the hub, or that the boat is trimmed incorrectly (where is the trim pin located in the transom shield? There are three holes and the default is the middle.) I would expect that with your current propeller your boat should run at around at 28 knots at WOT (just over 20% slip).
Is it possible for you to upload a picture of each of the propellers you are talking about? I would like to have a look at the blade size and shape of the prop before giving you a guesstimated speed figure. Remember that in addition of the pitch and diameter, there are other factors like cup, rake, skew, etc that have a significant influence in a propeller performance. In principle, I would not expect to see a significant increase in speed by going from a 15 x 17 to a 14 x 19, but the way the boat handles and the hole shot may be affected depending on the propeller shape."