A quick thought, I'm not sure exactly what version you have of Max, but have you tried to use a bezier float controller for the motion and adjust the bezier handles accordingly? This should affect only the motion and keep your pos. scale the same.
For example: take an object and assign a path constraint, select your path, then go into the curve editor with that object selected and right click your "percent" parameter on the left side. Choose assign controller and select a bezier float. This will give you handles to control the animation along the path with affecting the position scale on the path itself.