I have an HDS 9 on a RAM D size at the bow. Remember you get some additional height from the ball at the base, the mount at the top, and the gimbal bracket for the unit. I believe those extension arms are measured center to center of the sockets. I have an extension arm I think is 8". With all of the additions in height, it is REALLY tall! I do not think I would like 11.
As for ride, anytime I'm going to open water, I fold the arm down so the unit rests on the deck and clamp it there securely. I don't feel like chancing it standing up in the air so high when I'm about to pound some rough stuff.