--I use 9.9 on my 17.5 ft alum. Electric start so it charges my batteries for downriggers. Also found it was quieter than smaller 5hp honda 4 stroke and smoother. The higher hp on larger lakes just in case I need it to get back home if I have an issue with the main motor. I also carry a small gas tank so I can plug it in direct if there is a fuel issue...I usually run it off of the main tank the extra can is for emerg.
--Boat speed can be an issue especially in saltwater with tides...if you will be using in areas that run strong tides a 15 hp may be more appropriate...same weight as 9.9
--Can't remember for sure I think I get about 10 mph a wot with the 9.9, might be higher as not something I was concerned about as I mostly run in freshwater no tides. NO issues with the 9.9 propelling the boat in heavy winds if I had to motor home with it but for trolling, control in higher winds I have to use the main motor. I usually troll upwind or crosswind with the main and then drift downwind using the motor or bow mount electric for steering.