Yes, they are nice.

If you need longer shutter speeds (and don't stop experimenting at 13 seconds) then small apertures, but if that isn't enough then ND filters to cut down the light even further.

The effect you have chosen is very nice. With flowing water, depending on the shutter speed, we can get from an ethereal mist to frozen droplets, and all the effects in between.

If going for ethereal mist then very slow speeds are needed and I think it's helpful then to have something sharp in the scene that we can latch on to.

These are fine as they are. Other effects are possible according to taste!

I think you'll be pleased, and perhaps surprised too at how crisp pictures can be.

An excellent choice - high quality and compact.

My feeling is that at 10m with a 400mm lens at f5.6 the problem was more one of depth of field than sharpness. At that distance the DOF if focused accurately on 10m would be from 9.89m to 10.11m, so with the chosen subject most of it will be out of focus.

I would try again with a flat-on subject such as the face of a building. Focus very carefully and then see what the open aperture performance is like. Then check again at 2 stops down, say f8 on this lens.

If you're still not happy, let me know how much you want for the lens....

Neither do I. We can learn a lot by doing that.

Really good, superb shot!

I can assure you I was smiling nonetheless, hoping that the words alone would be sufficient...

Only if the intent is pointless and shallow....like everything else, these are techniques that can be used to good effect when the circumstances are right. IMHO of course, as always!

There are a few Voigtlander and Zeiss lenses for Pentax that look very interesting.