That's a small diameter prop there ...
I'd go to Overton's and buy a 15" pitch 3-blade Stiletto stainless prop and would try to get your RPMs to within 200 RPMs of max, with 1/2 - 2/3rds fuel and normal gear.
Boats only LOSE RPMs as the season progresses ... bottom paint gets dirty, boat gets heavier, you add more crap or fishing gear ['cuz that ain't crap

], and ALL normally aspirated motors will lost up to 200 RPMs during the heat of the summer.
Best thing ... Overton's will SWAP that prop out until you get one that works for you. Oh yeah, you have a good prop guy there, as usually they can only dial +/- 100-200 RPMs from blades as is.