@ Spaced Ranger
Very interesting indeed! The non-linear filter I used in the 2nd step indeed filters the lighter areas more strongly than the darker ones. Because I wanted the grain to more or less match the original grain in the GOUT, I filtered a standard upscale in the same way as the super resolution upscale. The grain I want to add is then the difference between the simple GOUT upscale and the filtered simple GOUT upscale.