prop pitch

Just wandering if anyone knows how much of a difference it would make if I were to change the prop pitch on my 15 hp mercury from 9 inch to 10.5 inch?

It is pushing a 16 foot alumacraft just tring to get a little more speed.

If your engine does not rev up to max recommended range of  rpm with a 9 pitch, the 10.5 will be even slower.
Correct prop pitch is determined by RPM
If the pitch is too low-you can over-rev the engine.
If is too high you won't reach proper RPM and lug the engine.
Diameter and shape change how it works as much as pitch does
Plastic, aluminum and stainless props all work differently also.
Find a shop that will let you test drive a prop

PGH hit it right with what he said.

This may or may not help. I had a 9.9 mer on my 14 foot aluminum boat, it has side console, floor... with a 9 pitch it did 8-9 mph, with a 7 pitch it did 8-9 mph with a 11 pitch it did 8-9mph. What i was facing was a motor that was not able to plane out this boat. And no matter what i did it would never achive what i wanted so i bought a motor that would. Now it is all smiles

Unless the motor can plane out your craft changing pitch (in my experences with lower hp motors)it not going to do much in terms of speed.