you can disable motion blur in render tags. this lets you seperate motion blur and no motion blur on object basis. then you could set the motion blur relatively low in render globals. and make (on object basis) in 3d motion tab the max visible samples higher. it might not change teh length (time wise) of the motion blur, but the quality of the edges. also note that according to manual you need highest quality anitaliasing if you want the max visible samples have an effect at all. the options in max might be better, i dont know max.

hope this helps