Skip the ND (Neutral Density) Filter and add your motion blur back in post.
Neutral Density Filters are a pain to use. From storing them, to attaching and removing them from the camera, to adjusting them to changing lighting conditions, they add significant hassle to video production. For the un-initated: we use ND’s to cut down on the amount of light entering the camera so we can force the aperture wider and keep that cinematic shallow focus in our shots.
That said, you will need the best cameras in order to achieve the perfect results using techniques such as this.