Difficulty entering the last zone.

So far I control every zone on the map with exception of the northern quadrant and have only entered the one race. My problem is, despite winning all the previous bosses cars, and setting them to the peak performance (selecting parts in MW was so much easier knowing nothing about cars), Im still being left on the line and struggling to reach anything past 5th place in the race wars.
After maxing out the DB9 it surpasses my other rides performance levels, but its still not enough to keep pace.

So even if you upgrade as high as you have unlocked, how are you meant to engage the other races and make progress?