Boated all day Saturday at Deer creek with new prop,it helped a lot was able to get to 5200 rpm and 21 mph gps it was pretty rough on the lake so might have done a little better in calm water. It is still sluggish takes 1 minute 10 seconds to get to full speed but I'm guessing that is because the motor is to small for the boat and weight.
What pitch? If 9 as suggested you got roughly a 2:1 improvement with a 30% change in prop pitch....yes 21 mph means you got up on plane {plane is defined as the boat on top of the water, with trailing water having moved away from the transom (transom is the vertical rear cross section of the boat) and forming a V out past the stern (stern is referring to the rear of the boat)}.
One minute ten seconds isn't all that bad considering your situation for a hole shot (hole shot defined as the time it takes to go from an idle in gear boat sitting essentially still, to the "on plane" condition defined above).
Now that you are on plane, don't expect such a drastic result from a slight change; won't happen....getting on plane is the big one.
Normally 200 rpm per inch of prop pitch change is a good rule of thumb. Don't have your engine specs but going to be 5500 or thereabouts so you have a little more to go and this will help your hole shot. Go on down to an 8 pitch and if you can't get that a 7. Your rpms should come on up but your speed may not; depends on lots of things. What you will get is better control in rough water, being able to stay on plane at a lower speed and not loose it and a faster hole shot. Engine will love you for it. Diameter of the prop will probably increase slightly with a pitch decrease but don't get hung up on that.....dia will remain "about" 10".
Once you get all that squared away and are out running best you can with your current "tucked in" trim position, start pushing it out, monitoring performance and how the boat responds until you find the "sweet spot" (sweet spot meaning that for any given throttle position, best speed/handling conditions) for that load, that day, those weather and wave conditions which will usually vary from outing to outing. Normally the smoother the water the farther you push it (the lower unit away from the transom) out, raising the bow, reducing the hull drag and in doing so, increasing your speed and rpms.
Last add some Sea Foam fuel treatment to your gas. Auto parts stores carry it and directions are on the can. It is well worth the money to keep it in your fuel. Will clean out any carbon that has built up over the life of the engine and engine will run stronger.