Actually the only cars that need special tuning are the ones you use for drags and speed challenges. The grip and drift cars will do fine with the packages,no need for special tuning them.
On the contrary, every race mode can benefit from specialized tuning. It's just getting up and being bothered to twiddle around with it that's hard. Each and every car can be improved, and each race differs. The amount of corners, and the nature of the corners varies every single time you race. Remember - corners and races are like fingerprints - no two are alike. So treat every single one like it's the first corner in an F1 race. Crucially and decisively.