Use the following cheats to get the desired effect!
Easy Money: you have to purchase the $35,000 '02 350Z Fairlady Nismo S-tune, after that turn around and sell it back to the dealership to get $37,300.
Easy Grip Battles: just treat it like a drift match. It will help you to take corners faster and complete the game easier.
Boss's car: when all the members of a crew are defeated, you will meet the Boss. To get his or her car and a new performance upgrade, just defeat the Boss.
Easy Tuning: for this cheat you cannot use special versions of cars. The Nismo Skyline or any others like that are tuned from the manufacturer. Also you are not able to tune cars you get from defeated Bosses.
Defeating D.K.: just use the Brake Drift on the entire track when racing against D.K.