First off, if you have the 200% log in bonus that instantly doubles your XP and Prize money Online and Offline.
Secondly, Offline you can win a Race with a Fiat, Viper or a LMP car. You will not get any more XP that the race awards, for it being Tuned or Untuned.
Last but not least, Online, after one of the newer updates they have a thing called "Performance Points Bonus". I've only seen this in the Seasonals. What this does is: If you can win the Race with a Car that has LOWER PP than the Race limits you to, you get a bonus of XP and Credits.
A perfect example of how to exploit this is to run the 620PP Supercar Festival. If you use the Chaparral 2J, tune it down to its lowest PP, (538PP I believe) I have won in one race almost 2mill in credits and not sure on the XP but its lucrative for 7-8 minutes of your time.