Dude using anything besides the R1 cars they provide you with is gonna make it hard for you to win. Believe me, I used a Koenigsegg for the R2 classes, and if it wasn't for the power that car had over the R2 cars, I would have lost because it can't corner as good as the tuned R2 cars. I would suggest just using the R1 cars they give you, you'll have less of a headache and it'll be fun tackling those turns at 120 mph+. More specifically I suggest using the (green?) Puegot R1 car they give you and just put lighter weight rims on it. You should win everytime, unless there is another Puegot in the race, which then you have to watch out for it. Just a tip
I just completed the R1 races tournaments yesterday, and when I finished it, I got so happy because I thought I beat the game... Ummm... nope, there's another R1 tournament with 13 races in it.. FML..
Anyways, here are some pics
My Subbie

Jumping is funn
Playing with some effects
The old skool M3 in white is sexyy
And my other Beamer with the wing and front clip, to help it corner since it sucks at cornering with all it's power..