I usually find that I can beat 90% of the professional league
with the C5R or the Viper with the right pit strategy and set up. The vette is tough to race because its so damn heavy but I like to race with American Thunder. I use the GT40 but it has a lot of understeer that takes a ton of tire wear to get to drive correctly..tough to beat the cars that don't use the tires as much. The Viper and the Calsonic Le Mans cars usually kick the snot out of everyone except the GT1 car. Both cars must be set up with max hp--the Calsonic must be used with the stage 4 supercharger. I've spent untold hours on the machine and can generally win 2nd place on first try on the professional level. My wife hates that I'm so into the game....I also have F12002 and F1 Career challenge that are equally fun. Try Gamefaqs.com
for tips. The Cobra is a lot of fun but doesn't cut once you start racing the race cars....seattle is definitely one of the most fun tracks.