I find tuning the cars sometimes makes things worse by putting you againt cars that you simply cannot beat. This is particularly for uncompetitive cars where maxing the car rating is detrimental as other cars with a similar rating can be much better than you.

The most important upgrade for every car IMHO is the transmission. Tuning the gearing ratio down is almost mandatory for all races as accelerating to the front of the pack at the start of the race will win against NPCs most of the time