You people that win this with a stock car or on your first attempt are clearly VERY good drivers, stop saying its "easy", cause it ain't... your just good. I'm an average driver (gold x 24, silver x 30, bronze x 6 on the tests) and it took me alot of work to win this even after using ever mod possible.
Tips: not sure who settings I based my off of but stiffing the springs and reducing the roll bars helped. Listen to tires carefully, if you hear squealing your losing traction and time adjust your line, braking points or suspension as needed. The first turn can be taken quicker if you brake in two sections, slow the car down on the straight, then accelerate to the apex with a touch more braking before taking off at full speed. When I first did this challenge I was trying to do all my braking before turning in but the corner tightens, so you can go faster in the first half of the turn without scrubbing speed. Also you can cut the tight left hander - notice that's not grass its just green painted road. For the last tight turn dropping down to 2nd gives you more acceleration, I was trying to coast thru in 3rd but that wasn't helping. I passed the A2 on entry to the final turn, winning by .3xx, easily the hardest race for me in GT5 to date.