Speed is irrelevant. As Mike stated, the max rpm range of most Mercruiser engines (including yours) is 4400-4800 rpm. If you are only getting to this range there is an issue.
Diameter is a somewhat fixed value as it's dictated by the gap between the blade tip and the hull, or in the case of an outboard or I/O (outdrive) between the blade tip and the anti-ventilation plate.
Blade count (3 blades vs 4 blades) and pitch are what you are going to tune to tweak max rpm.
I would use the stock diameter and start reducing pitch. Each dop in pitch of 1" should increase max rpm by around 200.
I would tune the prop with an empty boat with full fuel and water. Then load and re-test.
A dirty bottom, too many layers of hard paint, or being overloaded can all adversely impact performance.