Weight reduction is one , and right at the crank.( favoring the 9") Another is our (cant speak for others) has a larger lock up clutch for more reliability. Also the design for torque multiplication is a little different in the 9X11, than a stock design 12" . I do not believe that that the d-5's should be messed with because they are a very well performing piece from the get go and are never as effecient as in stock form. But thats me, as i am sure their are conflicting opinions. Anyway asked and answered