prop and gear change???????

[oldseaox]

Merc with a 150 power head on a 200 lower unit. Lower unit has ratio 1.87-1 and 150 lower unit had 2.00-1. WOT should be 5000 to 5600 but is 4800. Boat is 20.5 ft and weight is approx 3400-3500 with normal load. Hull and eng are 2950 advertised weight, dry.
Currently using a 15.25-15 alu prop (Vortex), plan to stay with alu.

Any Ideas???