Here's my $0.02 worth: One thing to keep in mind about fins is the level of the motor when running. Motors that are mounted low in the water will have all the fin in the water all the time. This will have the greatest effect, both on trim and on drag. Motors mounted higher may have some of the fin in the water, or may not have the fin in the water at all at high speeds. This is a major consideration when installing a fin!
I installed a Doel-Fin on a boat with a motor set fairly high several years ago. I was trying to improve time to plane, and the fin did help a lot. However, at high speeds the fin was just in the water. If I trimmed the motor out a bit, the fin came out of the water. This caused a BIG change in boat handling and sometimes started a "fin porpoise" where the fin would go back and forth in and out of the water. It was also a problem in some turns when one end of the fin came out and the other stayed in.
The boat was terrible with the fin and I took it off right away!
In another case, I used a fin on a boat with lots of weight in the back and a high-pitch prop that really struggled to plane. This was an I/O with a lot of power and a cavitation plate that was probably 3" below the bottom of the boat. This boat also liked to porpoise with very little trim out. The fin worked wonders at improving planing and minimizing porpoise. The fin transformed the boat from a real pain to something pleasurable.
It was hard to determine the effect of top speed because the boat was capable of some 80 MPH and you almost never wanted to do top speed!
Most manufacturers recommend mounting the motor with the cavitation plate level with the bottom of the hull. This means that if you put a straightedge on the bottom of the hull, it will line up with the bottom of the cavitation plate on the motor. You will see some outboards and many I/Os with a lower mounting than this. I've seen some single I/Os with the motor WAY down there! Some boats benefit from a higher engine mount, with the cavitation plate above the bottom hull line. This is most effective when you want to go fast, as less motor in the water means less drag and more speed. All of this impacts how and even if a fin will work.
My experience is the only way you will know is to try it on your boat. You can talk to others, but you can't really compare unless they have the same setup (same boat, same engine, same prop, same engine mounting height, same loading and weight distribution, etc.). You also have to consider what you want - some want top speed, some want to pull a wakeboarder slow, some want low planing speed or fuel economy. The good news is that it will only cost you $50 or so to find out, and you can always sell it on ebay if you don't like it...
