If I understand the question...
The cars available to race in Arcade mode are as tuned as the car comes from the dealership. An A class Corvette would run like the Corvette you bought w/o modifications. As well any of the S class cars run as they would had you just bought them w/o any upgrades.
The computer matches your power to weight ratio for your competition. If you race your garage cars in arcade mode your competiton will reflect this. Ie, if you are running a 280 hp Vitz chances are you'll be running against Lotus and Vettes, etc... If not Rally cars or something like that. If you race a 560 hp Evolution you'll find yourself running against Race tuned Vipers, Gt- Ones etc...
