Dan, here in the sunny south I generally see about half n half surface drive to outboard ratio, the long tails here are slowly going away due to the speed difference between them and the surface drive. I run a 23 hp Mud Buddy Lite Mini on a Modified Cackler ( no motor well ). Generally I get 25 mph unloaded and 21 mph with full load. The mini's are a lot lighter than the full frame motors, which is better for the type boats we are using. I am finishing up a Scaup, which I am putting a 27 hp on.
The Cackler had to have trim tabs, was uncontrolable without them. I am sure I lost some mph due to the down force caused by them, but I love the way the boat planes out. I built in wedges in the Scaup and hope I have enough down force with the wedges so I will not need other modifications.
I have hunted ducks since 1974 religiously. The only hunting I do except for spring turkey. Since I started using a surface drive, I would not have anything else for duck hunting. Crank it up at the house, no water pump to worry about. Get to the ramp, crank it up and let it warm up while you unstrap the boat and put on clothing. Again, no water pump to freeze up while hunting. Motor parts are easy to get and cheap if you need something. Down side, no reverse. Plus, I can haul booty through the milfoil here, full speed. Try that with an outboard.
The motors are pricey, I would probably look at a 18 hp mini for your boat. Again you may have to use wedges or trim tabs to help contol your boat. Who knows, you may get that majic 40MPH that everyone is talking about...