I have heard that the X dimension on the old Rockets was too low and added a lot of drag. I have 650h.p. (630 torque at 5300rpm on dyno) and barely get 75 with it, and I have to be trimmed almost as far as it will go without using the trailer mode to get there. I have heard some say they have seen 85+ with that much h.p.. We tried many props as well and have not seen much improvement. I am thinking about adding an IMCO 2" shorty lower to bring the X dimension up 2". Has anyone had any similar experiences with too low X and adding the shorty? How much difference can it make???

It can make a huge difference 10+mph

It really depends on what the X dimension is now. 1-2 inches below the bottom is fine for that speed, try to take some measurements first. Some boats need to have enough drive in the water so when you trim up the prop has enough bite. Sometime bowlift can give you more speed than drive height. It can be time consuming to get the perfect drive height.

Something else is going on, you should see 80 with that HP. I get 67 with 400HP and a light load. What prop? RPM? tab position? fuel? people?

Years ago on the old Sunsation forum befor it crashed there was a thread about shorty's. Improved speed a little but the handling of the boat was not very good and had a hard time keeping the prop in the water in any chop.