[Bushwacker]

The fin attaches to cavitation plate, so I doubt it'll ever be clear of the water, unless you have motor set really high or on a jack plate! It will not effect your ability to trim the motor out; it may help reduce porpoising, or may allow you to trim out more before that happens. As for different styles of fins, I've seen some, maybe like the one you mentioned, that are relatively narrow but extend further aft, and may generate some lift by capturing/deflecting some of the prop wash. In general a high aspect ratio airfoil (like on a sailplane) is most efficient, i.e., it produces the least drag for a given amount of lift; delta wings, like the Doelfin, are also pretty efficient. I'm not an expert in hydrodynamics, but I believe there are a lot of similarities to low speed aerodynamics, where the flow can be considered incompressible for all practical purposes.

I'd be surprised if a fin actually increased top speed, unless the boat was badly balanced (very stern heavy) to begin with!