How fast can a stock leaf rearend be?

11.50s w/440 duster(actually a 383 now(just put it in this morning) we hurt the 440 that's normally in it, it's at the machine shop getting fixxed) w/26-11.50s and an 8 1/4 w/no snubber not 60fting as well as you are bone stock /6 suspension. We do need to do something though because you r driving the car the whole time lol.
Jr.