Thing is, you can't drive Stars & Stripes in Camaro LM to begin with, since it's the prizecar for this series. He must have done it in something else first to win the Camaro LM.....
300HP is too much for these races!
Did Infineon in a stock 249HP Used Lexus SC300, not easy but tremendous fun! 200pt race too!
Seattle seen off in a stock 289HP New Lexus GS300, again tricky, but fun and another 200pts in the bag!
The Buick GNX (stock 276HP) has been able to win Laguna Seca too, though I've not tried that one yet!
300HP should murder the competition, and as for 600HP?

Do we hear pushover?
I guess it all depends on how you define "best car"?
If you're just after the gold trophies, don't care about racing the AI, A-spec points and want to blitz the competition then a high HP modified monster will do the job.
If you're after the satisfaction of driving and listening to a rumbling big-block while winning the race, then a stock Viper, Callaway, Camaro SS or Corvette should do the trick and earn you more than single digit A-spec points.
If you're after a challenge, want triple digit points and like the idea of a classic mixing it with the modern day American muscle cars, a classic like the Chevelle 454 SS, or Dodge Hemi Superbee will work nicely, just watch out for the wheelspin and speed maxing out at 120mph if there's a long straight! (Especially Seattle!)
If you're after true competition and the smug satisfaction of beating the AI with about 1/2 of their HP, and maximum triple digit points then try one of the cars I mentioned above, and have fun!