Engine performance/RPM's are related to the propellor size & pitch, as well as the trim of both the outdrive and hull,and fuel flow.
What are the recommended max rpm's from Merc? I'm not too sure I am not too sure I would want to run that engine over 4800 rpm's,they are costly to repair/replace.
I would suggest trim/load weight adjustment to get maximum speed, prop changes would be my second choice, depending on hole shot or speed, which ever you are looking for.