Each person have diff tunes but the rating is the same. So there is lot of version for tunes just need to find the one that gives the best time. You can try to do it yourself as well. A lot trial and error. I bought a few tunes then i tweaked it here and there to try.
Tune means installing the best part to get the rating u need. Tune is important to determine the car rating. Car rating determine which lobby you will be racing. Lobby will be divided by the car rating. For example: 1650 - 1655. So it is best to have a car rating 1655 in that lobby so u have higher chance to win the race.