That's very interesting, I haven't used the Arnold Noice denoiser before, but am about to test it out on a current project. I wonder if it can be used on a Terragen render if it only needs Z, normal and depth - if I remember correctly I think it needs those AOVS in a multichannel exr for it to work.
The Cornell box is probably a worst-case scenario as if you use such a wide area light to illuminate, it is going to cause a lot of low-frequency artifacts that a denoiser might struggle with. Maybe those renders could get a bit closer in terms of sampling and leave the denoise for a final polish.
On a recent project I just finished I got into the habit of giving compositors raw and denoised versions of the same render and they could mix as they saw fit.
I do have Neat Video and will give that a go, the only disadvantage is that you have to give it a smooth patch of noise to work on that is not always present in a 3d render.