This might not be possible given the way the Sharpness add-on works. It looks like there's a blur map that gets generated based on the effect size, and it takes noticeable time. It can't update that map fast enough to give a good 'live' experience with this current implementation.

OK. Shall we keep the issue open in case some dev gets inundated by a brain wave?

I can't dimmly remember that stippi described how he solved similar issues in WonderBrush 3. Something like threads being started and replaced by a new thread if the input data changes? The demo was quite cool, with some computational expensive effect (or layer blendings). He moved an object over the canvas and the effects got applied when there was CPU time. Very smooth...

Anyway, probably bigger fish to fry first...

Yes, let's leave it open. There could be an answer in Wonderbrush, or maybe right here in ArtPaint. The Blur addon works live so maybe it does something smarter.