Nice work! One point of advice - try to think about the defense more on corner entry (like you're doing) but less on corner exit (unless the attacking car is really pushing for moves like over/unders, or if the exit immediately sets up the next corner).
Basically, you're losing time by not using all of the track on exit. When you've defended the entry, your next goal is to get the smoothest and fastest exit possible, and start leaving them behind :)