What are the rpm's at WOT?
The rev range at WOT will be in the owners manual. You want to be in the higher end of that range.
If it's not hitting that upper range then you need to go down in pitch. The engine will have to much load on it not allowing it to rev out.
If it's over reving then you need to go up. The engine will be revving it's titts off before you hit WOT.
Each prop pitch should change the RPM by about 300 to 500 rpm. So by going down to the 10 from the 12 that you have on, should see an increase in rpm between 600 to 1000 rpm.
To give you an idea on the prop pitch, if you were to rotate the prop one rotation in a solid substance.....it's the distance the prop would move forward with one rotation.
So the 12 you have on will move 12inches forward in one rotation of the propeller. But because it's in water etc It won't in theory due to cavitation, weight of boat, water is easy to move etc.
So the 10 would move 10inches in one rotation. You'll notice the 12 has deeper blades than the 10.
The engine will spin the 10 easier than the 12 due to it travelling less distance in one rotation.
I hope that's easy to understand. I'm not very good at explaining techy stuff.
You will need to get hold of a tachometer though. Call the dealer for help.
IMO the 20 will not go as well as the 15.
Sorry to say but four strokes don't rev and produce power like 2 strokes do.
My bro in law found that out.
He had a 40 2 stroke and went to a 60 four stroke and tbh the boat went better with the 40.
It's a 4.45m vnose punt. Stessle I think.
I tried to convince him to get a 50 2 stroke but he went the four and was not impressed.