I think there are a few cars that are setup to have way too much rear brake in them. The Lambo in the challenge you are speaking about as well as the Lotus in one of the Top Gear Special Events feel as if all the brakes are in the rear of the car. It takes some getting used to, but the challenge with the Lambo is possible with no additional aids.
I was able to Silver it after 2 quick restarts and knowing what I could and couldnt do with the car in the first two or three corners. I came up 3 seconds short of the winner (which I think is a Ferrari F430), but made a few mistakes during the race and am sure if I try it again, it wont be near as difficult to get the gold. Just be smooth and take what the car and track gives you dont try and win the race in one corner. I love the changes theyve made to GT5 and it really rewards the smoother driver more than the series ever has in the past, IMO I LOVE IT and feel its 100x more realistic than any console sim Ive ever played!
I wonder if people with the DFGT (which I believe has a brake bias control) could dial out rear brake during some of these challenges? To be fair to PD, Ive never driven a Lambo or the Lotus, but I cant imagine they come from the factory with that much rear brake in the car. If you have a DFGT and you can make that adjustment during these races
they would be cake!