I rarely mod my cars past weight reduction, sport tyres and all motor mods. And these come after amature races. I found better opponents with sport tyres, I think the game decides your opponents based on tyres owned for the car. I.E. the american championships will pin you against a C5-R and the Oreca Viper... Also, I am finding a lot of little jewels that I would have never concidered great cars such as the MS Elise. With the MS Elise I can do the World Champ, Dream Champ, All Star and JGTC in Amature with sport tyres. Of coarse my Elise only has full weight reduction, all motor upgrades, Sport brakes, (no ballance controler), the top clutch, fly wheel and Adj tranny. This car is so well ballanced I dont touch the dif or suspension.