I wouldn't say Buescher "improved" as a driver. I just think RFK improved drastically as an organization, which finally allowed Buescher to showcase his ability. But yeah, obviously from a statistical standpoint, he improved quite a bit.
I'm tempted to say McDowell because he really impressed at a variety of tracks, but again, a lot of credit should go to the team.
So, which driver(s) cut down on mistakes, capitalized in clutch moments, and displayed more skill/speed than they have in the past? I would say those factors are the clear indicators of personal improvement.