[JJ_SO]

Since I have a Hoya too and a formattech 150 × 150 ...  The interesting part of those long exposures is to make moving objects blur (waves, grass, branches of trees) while solid objects remain clear and this comes always with wind-activity. So "...not notice sharpness loss..." can mean "while pointing the camera towards rocksolid objects", "while using them on classic vintage lenses, very sharp at their time" or simply you have the best technique shooting on tripod. And it's made of cast iron, too.

 

The picture above was made on a very windy day, with a travel tripod. I tried to shield the camera, but I can't tell how successful. Another picture I noticed a loss of sharpness I made at a river and the railway bridge was in use by freight trains, so the railings of it blurred.

 

Another thought: blocking light - how do they do it? how clear and "unpolluted" of particles are these filters? As well as bad UV-filters can change sharpness, why not the substances deivering the light reduction? if even a Nylon stocking in front of a lens can blur an image? I have no idea how big the impact is, but I think, as well as light distraction by the filter itself, those particles are also influencing sharpness.