It's a complex problem.
You may be running out of rpms before you run out of torque currently.
You should start by pitching up one of the current
props and seeing what happens.
From there, you will start to get a better understanding of top end.
Realize your low speed handling (bollard pull) will suffer as you pitch
As a data point, a single
pushes my very similar boat at 7.2 knots. However, I'm pitched low and could probably eek out another knot
or 2 if I had a higher pitch
. Not messing with any of that until the boat is 100% done though.