The brochure says 46 - 52mph with the 5.7 so you might be just a little low but since it's a cuddy it will have a little extra weight given all the extra stuff you store in them and at 3000lbs dry its not a lightweight to start with. 43mph isn't far off the advertising so the 4 blade 20" pitch prop might be a good idea but you will likely get more out of a set of Smart Tabs.

Altitude also plays an important factor. For example, I went out to a lake this weekend that was about 900ft higher than my local lakes and I was getting about 200 less RPM and 2 or 3 less MPH. I am never going to get the same performance out of my boat as the guys who boat at lakes that are only a couple hundred feet above sea level.