Thornhill is going to be a monster. I would hope for Earl Thomas, but he isn't that physical. Still, there will Pro Bowls in his future. Fuller is getting a chance to play to his strengths, which is a good thing, but it points out that he is a limited player. As you say, Mathieu played coverage all day. Eventually, that will be Thornhill's role. You mention Breeland (not to be confused with Breeland Speaks) who had a PD and a FR, but I did not notice him much. Perhaps that'a good thing, because Ward got picked on.
I was expecting significant improvement from the LB corps. The old defense was so complex that young players couldn't grasp a lot of it. This one is much more instinctive and plays to our athleticism. The downside is that it can get predictable. Still, like Fuller, Hitchens is much looser and looked like the player we signed. Damien Wilson also looked good. OTOH it's like Ragland and Lee never got on the field.
You did not mention the DL, which is a shame. They were very good. The Jags chose to double Clark and Jones as much as possible, but the other guys made plays. Okafor, Ogbah, Nnadi all had solid games. My heart went out to Okafor when he tipped the pass and came inches short of a pock-6. Obviously, Frank Clark did pull one down.
On the other side of the ball, Mahomes is now 4-0 against top-5 defenses, rating the Jags on 20018. Watkins had the most impressive Chiefs WR games since Stephone Paige had 256 yards and 2 TD in the first half. https://www.pro-football-reference.com/boxscores/198512220kan.htm