Ok, I tried another test with limiting by passes (PT+PT, same as my first test on page 1), but I left the Corona HQ denoiser on. Details in the file name.
The main difference I see in the type of noise, is that with HQ filtering off, there is the 'classic' type of CG noise. But with it on, the noise is a little more 'smudged', which I assume is the HQ denoiser trying to deal with a different type of noise. I may try another test with all denoising off.