One thing I have noticed is the effect on the damping you mentioned. A very slow key release seems to cause a slight buzz as the string vibration gets clipped by the damper. I really like this effect. Is it a new feature or have I just not noticed it previously?

It isn't a new feature, it has been implemented for quite a while now, with variations between instruments which might be what you have noticed...

One thing I have noticed is the effect on the damping you mentioned. A very slow key release seems to cause a slight buzz as the string vibration gets clipped by the damper. I really like this effect. Is it a new feature or have I just not noticed it previously?

I listened to your recordings, there is indeed a difference at the end. In your recording done with version 5, isn't the overall key release noise a bit louder than with version 4? Nevertheless, on an acoustic grand, even if you release the key slowly, there is still a noticeable noise produced by the jack going back to its rest position (actually, a slow release velocity in an acoustic piano has more effect on the damping than on the key release noise), so I find the noise at the end of your version 5 consistent with the rest of the piece.

In the Bluethner Pianoteq model, the key release noise loudness range w.r.t. Note Off velocity is 12 dB, which you may find not sufficient. However I would rather reduce the overall key release noise by 3 dB.

I originally produced a recording of Chopin's Berceuse on a C3 about 5 years ago (PTQ 3).
The significant changes I made to the default settings were :-
Key release noise +8dB.
Note on velocity curve Linear (Input=127 : Output=127, Input =1 : Output =0 (Silent)).
Note off velocity curve Linear (Input=127 : output=127, Input =1 : Output = 5).
I found these settings invaluable as a practice aid which encouraged me not to be too clumsy releasing keys too quickly when playing as quietly as possible.

When I tried playing the recorded MIDI file with the same velocity and key release noise setting with Bluthner Chamber Recording AB on PTQ5, the key release was very much more audible, especially after the closing chord.
OK so shame on me for being too lazy to re-record this piece from scratch, after all, its a different piano.
I re-recorded the final few bars but could not get rid of the clunkiness of the key release.
When I tried the same settings with the same instrument on PTQ4 the key release noise is much less.
I'll post the MP3's and MIDI file in the user area for comparison.
The key release noise after the final close is inaudible with PTQ4.