No shorcuts used. I've made videos of these, links are below. In the first qualifying race, I run about 1:15, so about 45 second head start. In the head start races, I run the first two laps about 1:10, getting drift points and having to slow up on the two drops near the end of the lap. On the last lap, I run about 1:05, so my lead should increase about 5 seconds, but instead it increases by 18 seconds (0:45 to 1:03) so there's some sort of stats bug or a bonus on your time. Even without the bug, my lead would increase by about 5 seconds every time.
Regarding the 100%, I run every race in each vertical list, getting all wins , successes, or gold medals.
I have version 1.33, and have the AI set to balanced. Manual shift and all the assists are off.
The AI on the dirt tracks are the easiest, on the race tracks they're difficult, and on the road courses almost impossible to win against. Takes me 1/2 hour to an hour to memorize a road course, and even then the AI are almost impossible. My strategy is to stay behind the pack for all but the last lap, minimizing my damage while they seem to bang each other up, then I go for it on the last lap, sometimes having to spin out the lead car.
Here are the links to the no escape races videos:
First head start race: