Mine was initially mounted on the transom to the right of the ladder (1st photo). I was getting a little “noise” on the screen, presumably from turbulence related to the step hull. I moved it to the lower step (2nd photo) and it really cleaned up the images.
As for speed, the SI works up to about 8 mph, if you have the Y cable the unit will automatically switch over to the “shoot through” transducer inside the hull, so at least your traditional sonar continues to function.

