A couple of years ago, I worked on the update of the grain module (Let's improve grain).
I agree with you that the current implementation has many pitfalls, and it doesn’t look particularly good, especially at small grain amounts.
In my free time, I’m working on much more physically accurate simulation. My primary source of inspiration is a recently published resolution-independent algorithm (https://hal.archives-ouvertes.fr/hal-01520260/file/Film_grain_synthesis_computer_graphics_forum.pdf).
I hope to come back to the DT grain module; for now, I am a little lost in the math of the problem. Maybe I should post some updates in the forum to stay motivated, and go forward in the project.