That's interesting.
How I am tuning my cars(to the class limit) is go for as much tire grip as I can fit first without breaking into the next class(unless I am taking a car from like D to B for example) then weight reduction, then brakes/downforce. If it is an older car I go for antiroll bars and suspension first as no matter how good your tires are if you have all the weight shifting so strongly in a corner you are going to lose traction.
Power is almost always the last thing I do. Usually I fill in the last few points of class index with upgraded clutch/flywheel/driveshaft as they typically don't up the index much and are great for getting more power to the road from of the default tuning. Exceptions to this power rule are the super minis with like 80hp or something which just boosting it to say 120hp can make a huge difference since you get WAY more hp/ton on a car that weighs 1500lbs with the cheap upgrades than a car that already has ~250hp and weighs close to 3000lbs.
I had a pretty easy time in Forza 3 picking my cars to max out for all the particular classes, but in Forza 4 I seem to be having a little trouble. I have some cars set up for F E D and C class but they all feel like I could find something better....Even my A class Ferrari 512 Testa Rossa feels like it isn't as good as the exact same setup I have in Forza 3 for the same class....all of my cars are from the 90s though, maybe that is why?
All the modern low-tier cars just feel terrible. Crappy suspension and made-to-a-budget default settings, just feels like you have to upgrade them WAY more to reach the same level of handling/cornering as the 90s versions. Sports cars not included of course.