I bought the Mavic set of ND filters when I bought the Air. I haven't used them yet, but saw a guy getting 1.5s exposures on an overcast morning. I like smooth water wave shots so in interested to try that out. I haven't had anything close to a no wind day yet to try.
its hard to tell, the mav is usally in continous focus and the focus range is small green box i just push the screen in the general area i want in focus and it seems to do pretty good, i think the nd filters are more for video and slowing down the shutter speed for video, i dont think 1/2000 is optimal for 4k video and they recomend much slower and since theres no aperture nd is the only way to get there, i dont shoot much if any video so i dont worrry about it, i dont mind 1/2000 for stills, i know it will freeze the shot anyways haha, i do havea set of nd as well and only used them once but i didnt like the color cast, i got the cheapies haha