I have been on Holiday, so I came into this late.
Was the Hull clean a sucess?
The only thing I would add is that if you have the Engine makers Handbook or can look up the information for your Make and model on line, I would try to see if you can reach the full rated RPM of the engine and check what the fuel rack position is at that point.
If the engine is unable to reach rated RPM either the engine power is down or the engine is being overloaded by too much pitch on the prop.
If on the other hand the engine easily reaches rated RPM with the fuel rack below maximum then the proppeller has either too little area or too small a pitch to absorb the rated engine power.
As the early discussion mentions, as a working fishing boat, the draft would have been greater and maybe she was set up for towing nets, lines or other boats which would require more power at a lower speed. As a working boat she would have been designed to go to Sea and work in any weather, unless you are a masochist I guess you won't be deliberatly sailing into storms. These factors would also suggest the original designed prop pitch would be much less than the ideal for leisure use.
In theory you could get a higher top speed, reduced Revs (and therefore fuel consumption) at all speeds and the engine would be running with higher torque which is always good. (High cylinder pressure gives better piston ring sealing and high exhaust temps give better combustion keeping the cylinder heads and piston rings clean for longer, thus increasing time between overhauls). However, you must always leave a good safety margin to avoid overloading the engine in bad weather, with a dirty hull and when the engine performance has dropped off a little.
I do not have any practical experience with such small boats, but I would think that with the engine at full RPM the fuel rack should be about 80% on a flat calm day, with the hull clean and the engine developing its rated power. However I would seek confirmation from the prop suppliers on that.
If you go ahead and change the prop, be aware that if you increase the prop diameter, you reduce the clearance between it and the Hull and this may cause severe vibration problems.
Also the prop should have a number of blades that is not directly divisible by the number of engine cylinders, i.e. a six cylinder engine should not have a 6 or 3 bladed prop, this is avoid the prop vibration being synchronised with the engine vibration.
Good luck and I hope you enjoy your time afloat once you have got everything fixed up the way you want it.
It is always better to ask a stupid question than to do a stupid thing.