Rhody here's some merc performance numbers from their website
http://www.mercurymarine.com/look_de...ecks&mercV6=14
With the exception of running 400 rpm below ideal, it sounds like you're getting nice performance.
If you were thinking you might get better numbers than the 4-cyl consider this thought. Your added weight is farther aft. It may be giving you a hair more bow rise causing you to push a little more water. Also, if you have the 15-1/4" dia prop it to may be pushing more water than a smaller wheel of greater pitch, maybe slightly reducing your rpm from that ideal 4800... Remember - I really don't know squat about this stuff - just been thinking about my own wheel a lot. That being said, I wonder if a smaller 4-blade wheel with similar pitch wouldn't lift your stern a bit & squeeze just a little more out of her.