I put one on my previous boat with a 35 Johnson. I picked up about 3 to 5 MPH by being able to trim it out but also because it puts the motor back a few inches from the transom which increases the bite of the prop. You can cheat a little on the alignment of the cavitation plate to the bottom of the boat. Because it is set back from the tramsom you can move the motor up maybe an inch or two from where you would have placed it because of the extra bite the prop gets from being back away from the transom.
I did not realize I could have done that and I still got a good bit more speed.
I really liked the unit, it is heavy duty and I enjoyed not having to run to the back to move the motor up and down.
SeaRay
Mark 1:17 ...I will make you fishers of men