I won Madrid with my Toyota 2000GT after a lot of tuning. But, this GT really drives nice now! There's just enough oversteer to throw it around a corner when needed. Otherwise, it just drives right around. But, this came after I spent hours trying to dial it in and memorizing the track and gear changes.
So, I figured I'd bring a grenade to a fist fight and decided to finish the rest of the seasonal with the Lamborghini Miura. The only thing I changed was the ballast and the transmission settings according to each track. Sure, I bought the adj. LSD, the adj. suspension, the adj. tranny, etc. But, I didn't need to tune anything except the tranny and ballast.
It's so easy to rack up to the max $20M that I'm now in it to win it. The money is just a function of the trial and error of tuning the car to win. So, I don't really care if I bring a car that will catch the lead AI by the 3rd lap or not, honestly.