That does seem rather slow. With a 15HP outboard I can easily get 16 knots from my 3.5metre SIB which probably weighs quite a lot more than your 4.2metre SIB. I get 13 knots with a 9.9HP.
I'm not sure what you mean by:
I did notice that using any more than 1/2 rpm caused the boat to be sucked back down and come off the plane.
Does this mean you are getting 13 knots at half throttle? Assuming everything is inflated to the correct pressure, the outboard has the right shaft length for your boat, and the prop is a reasonable diameter / pitch (should be stamped on it, worth posting details here) I am wondering if:
1. You are getting a problem with fuel starvation.
2. The prop hub is slipping.
3. The prop pitch is much too low (not sure about this one)
A rev counter would help with the diagnosis, but assuming you do not have one, when you increase above 1/2 throttle does it sound like:
1. The outboard bogs down / does not increase revs as much as you would expect?
2. The revs seem to rise rapidly with no increase in speed?
The best way I know to check for fuel starvation is to run at full throttle for as long as possible, then pull the killcord without closing the throttle. You can then pop the spark plugs out and check the colour. The tricky bit is finding somewhere you can do this safely - it is a lot easier with a motorcycle!
Hopefully someone can suggest a simpler / safer approach.