Phil,
I have a 15 Yamaha on mine and it's plenty. The motor came with a 9.25/11 prop and I put a tach on it and it would only do 5000 rpms. SO, I put a 9.25/9 on it thinking it would run about 5500, but without a heavy load she'll do 6500 plus. I have to be careful not to red line her, but she really runs smoothly through rough water 9.25/9.
I can still cruise on plane with a load at about 14 or 15 mph land speed.
With a 9.25/11 I did 23 mph one day, but she's rides really rough with that pitch and it's a little hard on the motor (recovery etc).
Now, let me voice my opinion on the safety factor. That boat is rated by the USCG at 15hp for a reason. Need I say more? It's also rated at 965 lbs weight capacity. Would you put 1600 lbs in her and leave the dock?
I'd put a 15 on her with a 9.25/10 prop and see how she does.
Feel free to pm me if you want more performance info.
Dave