OK I got it: denoising simply comes from resizing the file from 4k to 700 x 700.
Here a 4h35 rendering - SL13 - Mac Pro 2.66 x 8 Xeons:
You still have to render to a sufficient SL: notice the highlight at the base of the sphere at SL13, missing from benjamin's picture at SL9.
But the resizing method seems to be the only way to denoise difficult images like this one (glossy/transparent surfaces everywhere), as the noise otherwise never really disappears (see Torolf's rendering at SL25!).