I think the skill ceiling and skill floors are different depending on the mod of choice, and even more so for replay making. But if we are talking classic comp mods I think we are probably closest to the skill ceiling of abd rather than mods like boxshu or lenshu, because the opener matters so much at the higher level it seems like the mod is much more limited than the other ones with what works and what doesn't at a high level of play. I think brown/black belt is when people start to know what they are doing a bit and 7th-8th dan is when you start being able to hold your own with better players a bit more, but will still get smashed by top players. 10th dan to master belt is where you really get good and then custom and later is where you maximize skill and you start to plateau if you don't focus on improvement.
I think belts can be a measurement for skill if the qi matches the games played, games played is really what determines skill, obv everyone learns at their own pace, but if you have 10k games in a mod, I would think you would be competent in it.