If your motor has enough oomph to plane it off, you want the higher pitch prop (9-1/4 x 8) for max speed because more pitch = more speed, and on a small boat like that more pitch keeps the RPM from revving too much above the manufacturer’s spec., assuming it doesn’t load the RPM way down too much so it doesn’t plane off. Pitch is the prop’s second number, and it is how many inches the prop pushes the boat with each revolution, assuming ideal case – no slippage, etc, which never really exists in the real world. One inch pitch increase is like going from 3rd to 4th gear on a car.

A motor set up for racing would have a 2-blade prop with small diameter (the first number on the prop) both items reduce drag; along with high pitch, and a higher gear ratio (lower ratio number) to further transmit power with out over-revving. Racing versions also have no gearshift so the lower unit can be made really thin for less drag.

The site below claims 25 MPH with a 9.9, but I’d guess that’s for a 70 pound kid in it. It may not plane off with a 200 pound adult.

http://muskokaseaflea.ca/StaticWeb2/?page_id=259

It was designed to be a class B hydro, and that would be a 20 cubic inch motor (which your 9.9 is), with racing lower unit. putting out all it can – which would be a 15 HP OMC, not the 9.9. Thant’s still 20 cu in, but a bigger carb.
Dave